Technical Field The present invention relates to a process for production of diacylglycerol fat/oil in which a polyunsaturated fatty acid is a constituting fatty acid. It further relates to a process for production of fat/oil containing a high amount of triacylglycerol and a low amount of diacylglycerol or fat/oil containing a low amount of triacylglycerol and a high amount of diacylglycerol by a fractional distillation of fat/oil which contains triacylglycerol and diacylglycerol. It still further relates to said fat/oil and to food and drink, to nutritious food for therapy, animal feeds, pet food and to drugs in which said fat/oil is compounded. In the present invention, the triacylglycerol and the diacylglycerol are those which are extracted from microbes. Background Art The term of a polyunsaturated fatty acid used here stands for a fatty acid having 18 or more carbons and having two or more double bonds. Since a polyunsaturated fatty acid has various unique physiological activities, it is used for the enhancement of functions by adding it to various kinds of foods and animal feeds. Main examples thereof are linoleic acid (LA), oc-linolenic acid (ALA) , y-linolenic acid (GLA) , dihomo-y-linolenic acid (DGLA), mead acid (MA), arachidonic acid (AA) , eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and so on. In its utilization, although it is used in a free fatty acid type or a phospholipid type, it is mostly used in a triglyceride type and there are many cases where polyunsaturated fatty acid is contained, as a constituting component, in its acyl residues. In biosynthesis of human polyunsaturated fatty acids, there are two representative types, co3 type and co6 type, (co shows numbers counting from the terminal methyl group of the fatty acid to a carbon where the closest double bond is present) and, in the case of an co6 type for example, unsaturation and carbon chain extension are repeated from linoleic acid (18:2 co6) to transform to ylinolenic acid (18:3 co6) , dihomo-y-linolenic acid (20:3 co6) , arachidonic acid (20:4 co6) and 4,7,10,13,16- docosapentaenoic acid (22:5 co6). Similarly, in the case of an co3 type, unsaturation and carbon chain extension are repeated from a-linolenic acid (18:3 G>3) to transform to eicosapentaenoic acid (20:5 co3), 7,10,13,16,19-docosapentaenoic acid (22:5 co3) and 4,7,10,13,16,19-docosahexaenoic acid (22:6 o3). Among the polyunsaturated fatty acid of an co3 type, eicosapentaenoic acid (hereinafter, referred to as "EPA") and docosahexaenoic acid (hereinafter, referred to as "DMA") have been particularly known to have many physiological functions such as preventive effects for adult diseases such as arteriosclerosis and thrombosis, anti-cancer action and enhancing action for learning ability and various attempts have been carried out for their utilization to drugs and designated health foods. In recent years however, attention has been also paid to physiological mechanisms of other types of polyunsaturated fatty acid than the co3 type (co6 and co9 types). Arachidonic acid occupies about 10% of fatty acids which constitute blood and important organs such as liver (for example, the fatty acid composition ratio of phospholipid of human blood is 11% of arachidonic acid, 1% of eicosapentaenoic acid and 3% of docosahexaenoic acid), participates in adjustment of the fluidity of a membrane as a main constituting component of cell membrane and shows various functions in metabolism in vivo and, on the other hand, it also plays an important role as a direct precursor for prostaglandins. Particularly in recent years, it has been receiving public attention as nutritional supplement for babies and small children as well as for aged people. Usually, when food abundant in linoleic acid is ingested, it is transformed into arachidonic acid but, in patient suffering from adult diseases including people who are about to suffer the diseases, babies/small children and aged people, activities of enzymes participating in biosynthesis lowers and arachidonic acid is apt to become deficient whereby it is desired to directly ingest it as a fat/oil. As to EPA or DHA which are polyunsaturated fatty acids of an o>3 type, there is an abundant supply source therefor which is fish oil but about y-linolenic acid, dihomo-y-linolenic acid, arachidonic acid and 4, 7, 10, 13, 16-docosapentaenoic acid (22:5 0)6) which are polyunsaturated fatty acids of an co6 type, they are rarely available from conventional supplying sources for fat/oil and, at present, fat/oil where polyunsaturated fatty acid prepared by fermentation of microbes is a constituting fatty acid has been generally used. For example, there has been a proposal for a method where various microbes which are able to produce fat/oil in which arachidonic acid is a constituting fatty acid are incubated whereupon fat/oil having arachidonic acid as a constituting fatty acid is produced. Among the above, it has been particularly known that fat/oil containing high amount of arachidonic acid is produced using a microbe of genus Mortierella (Japanese Patent Laid-Open No. 63/044,891 and Japanese Patent Laid-Open No. 63/012,290). The fat/oil mainly contains triacylglycerol. Incidentally, diacylglycerol is also widely present in natural fat/oil. Diacylglycerol has been receiving public attention because it has an action to suppress a rise in neutral fat in blood. Recently, its industrial production by an enzymatic synthetic method has become possible and diacylglycerol has been receiving public attention as food for suppressing a rise in neutral fat in blood and has been utilized as well (Kagaku to Kogyo, 74(1), page 33 (2000)). It has been utilized in the field of cosmetics, and drugs, and so on. Suzuki, et al. (Yvkagaku, volume 31, no. 11 (1982), page 921) show that, when Mortierella isabellina IFO 7884 is incubated using ammonium carbonate as a nitrogen source, a neutral fat containing 35% of 1,3- diacylglycerol can be produced. However, a fatty acid composition contained in the diacylglycerol, as such, has not been disclosed. Further, Suzuki, et al. (Yukagaku, volume 37, no. 11 (1988), page 1081) show that, when dried cells prepared by incubation of Mortierella isajbellina IFO 8187 is extracted with ethanol which is a polar solvent, diacylglycerol occupying 66.6% of neutral fat is produced. However, no fatty acid composition in the diacylglycerol, as such, is disclosed. In addition, in that method, the polar solvent is also extracted in large quantities and, in order to raise the purity of the neutral lipid, it is further necessary to remove the polar lipid. Moreover, as a result of investigation of extracting conditions, the fat/oil is that where diacylglycerol is concentrated and is not fat/oil per se produced by the microbe. Shimizu, et al. (Biotechnology in Agriculture and Forestry, vol. 33, Medical Aromatic Plants, VIII, page 360) show that a mutant strain of MortierelJa alpine 1S-4 produces fat/oil containing 20% of diacylglycerol and the amount of arachidonic acid produced therein is 19.4%. Incidentally, diacylglycerol is also widely present in natural fat/oil. It has been utilized in the field of cosmetics and drugs and so on, moreover, in recent years, a method for producing the same in large quantities and also in high purity has been found, its nutritional study has made a progress. As a result, its utilization as a food for suppressing a rise in neutral fat in blood has been conducted (Kagaku to Kogyo, 74(1), page 33 (2000)). With regard to a method for an industrial production of diacylglycerol using enzyme or the like, it has been disclosed as follows. For example, in Japanese Patent Laid-Open No. 01/071,495, diacylglycerol is produced from glycerol and lower alcohol ester of fatty acid using an immobilized lipase whereupon fat/oil where diacylglycerol is 80.1% is produced. Moreover, in Japanese Patent Laid-Open No. 62/025,987, 60 to 70% of diacylglycerol is produced from fatty acid, lower alcohol and glycerol using an alkaline lipase under the condition where no water is substantially present. However, for the diacylglycerol as such, it is produced using enzymes. Further, no process for production of diacylglycerol containing a polyunsaturated fatty acid has been found yet. Thus, fat/oil which contains a polyunsaturated fatty acid such as arachidonic acid and further contains diacylglycerol in a high concentration has not been reported yet and it has not been industrially manufactured and its composition has not been known at all. So, its production from the natural world such as microbes without chemical modification of enzyme has been entirely neither known nor carried out. With regard to microbial fat/oil where triacylglycerol contains arachidonic acid in a high concentration, its production process and the fat/oil per se have been disclosed already but, with regard to microbial fat/oil where amount of triacylglycerol is high while amount of diacylglycerol is low, a production process has not been disclosed yet. It has been receiving public attention in recent years that diacylglycerol has an action to suppress the rise of neutral fat in blood. However, in view of stability against oxidation, fat/oil containing less monoacylglycerol is preferred. Thus, as compared with a product where diacylglycerol is highly purified, that where triacylglycerol coexists is preferred. Suzuki, et al. (Yukagaku, volume 37, no. 11 (1988), page 1081) show that diacylglycerol occupying 66.6% of neutral fat is produced but, in the report, dried cells are extracted with ethanol which is a polar solvent and, according to this method, polar lipid is also extracted abundantly whereby it is necessary to further remove the polar lipid in order to improve the purity of the neutral lipid. Moreover, the fat/oil is that where diacylglycerol is concentrated as a result of investigation of extracting condition and is not the fat/oil per se which is produced from the microbe. In Japanese Laid-Open Patents No. 01/071,495 and Nos 62/025,987, the product is produced in an industrial scale using enzymes. Incidentally, no process for production of diacylglycerol containing a highly unsaturated fatty has been found yet. Shimada, et al. (LIPIDS, vol. 31, no. 12 (2003)) conducted a transesterification reaction between arachidonic acid-containing triacylglycerol and ethanol using an enzyme whereupon, during the reaction, around 25% of diacylglycerol was by-produced but monoacylglycerol was also by-produced to the similar extent. Thus, a fat/oil which contains a polyunsaturated fatty acid such as arachidonic acid and further contains diacylglycerol in a high concentration has not been reported yet and, nothing to say, it has not been industrially manufactured and its composition has not been known at all. So, its production from natural world such as microbes without chemical modification of enzyme has been entirely neither known nor carried out. With regard to microbial fat/oil where triacylglycerol contains arachidonic acid in a high concentration, its production process and the fat/oil per se have been disclosed already but, with regard to microbial fat/oil where amount of triacylglycerol is high while amount of diacylglycerol is low, its production process has not been disclosed yet. Although microbial fat/oil containing diacylglycerol has been reported already, the production method therefor is not suitable for an economical production because concentration of diacylglycerol is low. Under such circumstances, it has been aimed to develop a microbe by which fat/oil containing 21% or more diacylglycerol in neutral lipid is able to be produced. Patent Document 1: Japanese Patent Laid-Open No. 63/044,891 Patent Document 2: Japanese Patent Laid-Open No. 63/012,290 Patent Document 3: Japanese Patent Laid-Open No. 01/071,495 Patent Document 4: Japanese Patent Laid-Open No. 62/025,987 Non-Patent Document 1: Kagaku to Kogyo, 74(1), page 33 (2000) Non-Patent Document 2: Yukagaku, volume 31, no. 11 (1982), page 921 Non-Patent Document 3: Yukagaku, volume 37, no. 11 (1988) , page 1081 Non-Patent Document 4: Biotechnology in Agriculture and Forestry, vol. 33, Medical Aromatic Plants, VIII, p. 360 Non-Patent Document 5: LIPID, vol. 31, no. 12 (2003) Disclosure of the Invention For the purpose of obtaining a microbe which can produce fat/oil containing 21% or more diacylglycerol in neutral lipids, the present inventors have prepared many mutant strains of Mortierella alpine and selected mutant strains which produce fat/oil containing high concentration of diacylglycerol from them. The result was that, to our surprise, when glucose concentration and incubating time are optimized, a process for production of fat/oil in which polyunsaturated fatty acids where diacylglycerol is 30% or more and triacylglycerol is 50% or less to neutral lipid are constituting fatty acids has been achieved. It is also possible that the fat/oil in the microbial cells is appropriately extracted with a solvent and then separated using a known technique such as fractional distillation whereupon fat/oil containing a higher amount of diacylglycerol and a lower amount of triacylglycerol than the intracellular fat/oil is prepared. One of the fat/oil separated by that method contains a high amount of triacylglycerol and it is possible to prepare fat/oil containing a low concentration of diacylglycerol. Accordingly, the present invention provides a process for production of fat/oil containing a high amount of diacylglycerol and also provides food and drink, nutritious food for therapy, animal feeds, pet food and drug in which said fat/oil is compounded. Accordingly, in a process for production of diacylglycerol-containing fat/oil in which polyunsaturated fatty acid is a constituting fatty acid and ratio of diacylglycerol to the total neutral lipid is more than 20% by weight or more, the present invention provides a process which is characterized in that a microbe which is able to produce said fat/oil is incubated and, if desired, said fat/oil is collected therefrom. In the above-mentioned process, it is preferred that the ratio of diacylglycerol to the total neutral lipid is not le ss than 30% by weight or, more preferably, the ratio of diacylglycerol to the total neutral lipid LS not less than 40% by weight and, for example, the ratio of diacyl glycerol to the total neutral lipid is not less than 5 0% by weight. In the above-mentioned process, the aforementioned microbe is preferably that belonging to genus Mortierella and, more preferably, that belonging to subgenus Mortierella of genus Mortierella such as a microbe of species Mortierella alpina. Preferably, the aforementioned microbe is a variant of a microbe which is able to produce fat/oil containing a polyunsaturated fatty acid. The present invention also provides a microbe of genus Mortierella which is able to produce diacy^Lglycerol-containing fat/oil where a polyunsaturated fatty acid is a constituting fatty acid and the ratio of diacyl_glycerol to the total neutral lipid is more than 20% by weight or, preferably, a microbe of genus WbrtiereJJa which is able to produce diacylglycerolcontaining fat/oil where a polyunsaturated fatty acid is a constituting fatty acid and the ratio of diacylglycerol to the total neutral lipid is not less than 30% by weight. Preferably, the aforementioned microbe is Mortierella alpina and is, for example, a mutant strain thereof. The present invention further provides microbial cells containing diacylglycerol-containing fat/oil where a polyunsaturated fatty acid is a constituting fatty acid and the ratio of diacylglycerol to the total neutral lipid is more than 20% by weight. Preferably, the ratio of diacylglycerol to the total neutral lipid is not less than 30% by weight and, more preferably, the ratio of diacylglycerol to the total neutral lipid is not less than 40% by weight. For example, the ratio of diacylglycerol to the total neutral lipid is not less than 50% by weight. The above-mentioned microbe is preferably microbial cells of a microbe belonging to genus Mortierella and, for example, it is microbial cells of a microbe of species Mortierella alpina. Microbial cells are sterilized, living or dried. The present invention still further provides diacylglycerol-containing fat/oil where a polyunsaturated fatty acid is a constituting fatty acid and the ratio of diacylglycerol to the total neutral lipid is more than 20% by weight where said fat/oil is extracted with incubated cells of a microbe which is able to produce said fat/oil. Preferably, the ratio of diacylglycerol to the total neutral lipid is not less than 30% by weight and, more preferably, the ratio of diacylglycerol to the total neutral lipid is not less than 40% by weight. For example, the ratio of diacylglycerol to the total neutral lipid is not less than 60% by weight. For example, the above-mentioned extraction is carried out using a nonpolar solvent or a hydrophilic solvent. The present invention furthermore provides fat/oil containing not less than 70% of diacylglycerol and not more than 30% of triacylglycerol in a neutral lipid prepared by fractionation by extraction and distillation of fat/oil from cells containing diacylglycerol and triacylglycerol prepared by incubation of a microbe which is able to produce fat/oil comprising a polyunsaturated fatty acid as a constituting fatty acid. The present invention still furthermore provides fat/oil containing not less than 95% of triacylglyceride and not more than 5% of diacylglycerol in a neutral lipid prepared by fractionation by extraction and distillation of fat/oil from cells containing diacylglycerol and triacylglycerol prepared by incubation of a microbe which is able to produce fat/oil comprising a polyunsaturated fatty acid as a constituting fatty acid. The polyunsaturated fatty acid constituting the fat/oil of the present invention is, for example, dihomoy- linolenic acid (20:3 0)6), arachidonic acid (20:4 0)6), 1, 10,13, 16-docosatetraenoic acid (22:4 cc>6) , 4,7,10,13,16- docosapentaenoic acid (22:5 006), 6,9,12,15- octadecatetraenoic acid (18:4 co3) , 8,11,14,17- eicosatetraenoic acid (20:4 co3) , eicosapentaenoic acid (20:5 co3), 7 , 10 , 13, 16,19-docosapentaenoic acid (22:5 o>3), 4 ,17,10, 13, 16, 19-docosahexaenoic acid (22:6 co3) , 6,9- octadecadienoic acid (18:2 co9) , 8, 11-eicosadienoic acid (20:2 co9) or 5, 8, 11-eicosatrienoic acid (mead acid: 20:3 o9) or a combination thereof. The present invention also provides food composition, animal feeds composition, pet food, materials for chemicals, drug and cosmetic composition where the above-mentioned fat/oil of the present invention is contained or the fat/oil is used as a material or is chemically modified. Best Mode for Carrying Out the Invention Preparation of mutant strain The present invention relates to food and drink, nutritious food for therapy and drugs compounded with pure fat/oil which is prepared in such a manner that a microbe which is able to produce fat/oil in which amount of a polyunsaturated fatty acid where diacylglycerol in fat/oil, specifically in crude oil, is high as a constituting fatty acid is incubated to produce said fat/oil and then said fat/oil is purified. Accordingly, in the present invention, any microbe is able to be used so far as it is a microbe which is able to produce fat/oil (diacylglycerol) where a polyunsaturated fatty acid is a constituting fatty acid. For example, with regard to a microbe which is able to produce fat/oil (triglyceride) where arachidonic acid is a constituting fatty acid, microbes belonging to genus Mortierella, genus Conidiobolus, genus Pythium, genus Phytophthora, genus Penicillium, genus Cladosporium, genus Mucor, genus Fvsarium, genus Aspergillus, genus Rhodotorula, genus Entomophthora, genus Echinosporansium and genus Saprolegnia may be listed. With regard to a microbe belonging to genus Mortierella and subgenus Mortierella, examples thereof are Mortierella elongata, Mortierella exigua, Mortierella hygrophila and Mortierella alpina, and so on. Specific examples thereof are strains of Mortierella elongata IFO 8570, Mortierella exigua IFO 8571, Mortierella hygrophila IFO 05941, Mortierella alpina IFO 8568, ATCC 16266, ATCC 32221, ATCC 42430, CBS 291.35, CBS 224.37, CBS 250.53, CBS 343.66, CBS 527.72, CBS 529.72, CBS 608.70 and CBS 754.68. For example, with regard to a microbe which is able to produce DHA, a microbe belonging to genus Crypthecodenium, genus Thrautochytrium, genus Schizochytriim, genus Ulkenia, genus Japonochytriiw or genus Haliphthoros may be listed. All of those strains are available without any restriction from the Institute for Fermentation, Osaka (IFO), the American Type Culture Collection (ATCC), U. S. A. and the Centraalbureau voor Schimmelcultures (CBS). It is also possible to use Mortierella elongata SAM 0219 (Accession Number 8703 at the Fermentation Research Institute) (Accession Number 1239 at the Fermentation Research Institute according to the Treaty) which is a strain separated by the study group of the present invention from the soil. Although the strains belonging to those type cultures or the strains separated from nature may be used as they are, it is also possible to use a natural mutant prepared by one or more growth and/or separation operation(s) and having different properties from those of the original strains. Thus, when the lipid-producing microbe is subjected to a mutation treatment and selected, it is also possible to select a lipidproductive microbe having an enhanced productivity of diacylglycerol. Although there is no particular limitation for the mutation treatment so far as it is applicable to the above-mentioned lipid-producing microbe, an example is a common mutation treatment such as irradiation with radioactive ray (X-ray, gamma-ray and neutron ray) , irradiation of ultraviolet ray, treatment at high temperature and a method where a microbe is suspended in an appropriate buffer or the like, mutagen is added thereto, the mixture is incubated for a predetermined period, appropriately diluted and inoculated to an agar medium to give colonies of a mutant strain. Examples of the mutagen are alkylating agents such as nitrogen mustard, methylmethane sulfonate and N-methyl-N' -nitro-Nnitrosoguanidine (NTG); base analogs such as 5- bromouracil; antibiotics such as mitomycin C; inhibitors for base synthesis such as 6-mercaptopurine; dyes such as proflavine; certain types of cancer-causing agent such as 4-nitroquinoline-N-oxide; and compounds such as manganese chloride and formaldehyde. The microbe used may be either growing cells (hyphae) or spores. Although there is no particular limitation for a method by which a mutated lipid-producing microbe having an enhanced productivity for diacylglycerol is selected a mutated lipid-producing microbe, it is preferred that lipid which is produced by a mutated lipid-productive microbe is analyzed by a high-performance liquid chromatography or the like. As shown in Example I, when about 3,000 cells of the strain subjected to mutation were selected as mentioned above, three strains of mutants which produce diacylglycerol-containing fat/oil where the ratio of diacylglycerol to the total neutral lipid is more than 20% by weight and a polyunsaturated fatty acid is a constituting fatty acid were obtained. That means one strain in average of aimed mutant strain was obtained per 1,000 mutated strains. Accordingly, frequency of preparation of the mutant strain of the present invention is far higher than the case of a random selection by conventional mutation and selection and the mutant strain having the above-mentioned characteristic similar to the three strain mutants actually prepared in the present invention is able to be easily prepared by repeating the method mentioned in Example 1 of the present invention. A microbe belonging to genus Mortierella and subgenus Mortierella has been known as a microbe which is able to produce fat/oil (triacylglycerol) where arachidonic acid is a main constituting fatty acid and, as a result of subjecting the above-mentioned strain to a mutation treatment, the present inventors have prepared a microbe which is able to produce fat/oil (triacylglyceride) where dihomo-y-linoleic acid is a main constituting fatty acid (Japanese Patent Laid-Open No. 05/091,887) and a microbe which is able to produced fat/oil (triacylglyceride) where an 06 type polyunsaturated fatty acid is a main constituting fatty acid (Japanese Patent Laid-Open No. 05/091,888). They have also prepared a microbe having a resistance to carbon source in a high concentration (WO 98/39468). Those microbes are microbes belonging to genus Mortierella and subgenus Mortierella and, when those strains are subjected to the same mutating treatment as in the present invention, it is possible to prepare microbes which accumulate fat/oil having a high amount of diacylglycerol where dihomo-y-linolenic acid or o>9 type polyunsaturated fatty acid as a main constituting fatty acid. The fat/oil of the present invention is a microbial fat/oil which is prepared from an incubated product obtained by incubation of a microbe which is able to produce fat/oil where a polyunsaturated fatty acid is a constituting fatty acid. Thus, as a result of incubation of a microbe in an incubation tank, fat/oil containing a high concentration of diacylglycerol in the cells is accumulated and then said fat/oil is extracted therefrom. To be more specific, it is a fat/oil containing not less than 21% by weight or, preferably, not less than 35% by weight of diacylglycerol to the fat/oil and containing not less than 2% by weight or, preferably, not less than 9% by weight of a polyunsaturated fatty acid to the total fatty acids in diacylglycerol. Accordingly, it is essential to incubate a microbe which is able to produce a fat/oil (diacylglycerol) where a polyunsaturated fatty acid is a constituting fatty acid. With regard to the microbe used here, it is preferrably a microbe which produces at least one of an co6 type polyunsaturated fatty acid where carbon numbers are 18 or more and double bonds are 3 or more, an 0)9 type polyunsaturated fatty acid where carbon numbers are 18 or more and double bonds are 2 or more and an 6) , 4 , 7, 10, 13, 16-docosapentaenoic acid (22:5 eo6) , 6,9,12,15- octadecatetraenoic acid (18:4 co3) , 8,11,14,17- eicosatetraenoic acid (20:4 co3) , eicosapentaenoic acid (20:5 co3), 7, 10, 13, 16, 19-docosapentaenoic acid (22:5 co3 4, 7, 10, 13, 16, 19-docosahexaenoic acid (22:6 co3) , 6,9- octadecadienoic acid (18:2 co9), 8, 11-eicosadienoic acid (20:2 co9) or 5, 8, 11-eicosatrienoic acid (mead acid: 20:3 co9) or a combination thereof. 28. A food composition, an animal feeds composition, pet food composition, a material for chemical products, a drug and a cosmetic composition in which the fat/oil mentioned in any of claims 19 to 27 is contained, is used as a material or is chemically modified. 29. A Process for producing diacylglycerol containing Fat/oil/ substantially described herewith foregoing description and examples.