HYALURONIC ACID DERIVATIVE GEL AND METHOD FOR PREPARING THE SAME FIELD OF THE INVENTION The present invention relates to hyaluronic acid derivative gels, more particularly hyaluronic acid derivative gels which are formed by coupling an amine group-containing saccharide compound, having a variety of molecular weights, to a hyaluronic acid, having a variety of molecular weights, or a cationic salt thereof, via amidation reaction, and a method for preparing the same. The hyaluronic acid derivative gels according to the present invention have various different properties to heat, depending upon the amidation reaction condition and additional heat treatment BACKGROUND OF THE INVENTION Hyaluronic acid is a linear biocompatible polymer comprising linked repeating units of N-acetyl-D-glucosarnine and D-glucuronic acid, which is present in high concentrations in the vitreous body of the eye, the synovial fluid of joints, rooster comb, etc. As used herein, the term "hyaluronic acid" sometimes refers to both hyaluronic acid and any of its cationic salts. The cationic salt of hyaluronic acid used in the present invention includes such inorganic salts as sodium hyaluronate and potassium hyaluronate and such organic salts as tetrabutylammonium hyaluronate, but is not limited thereto. Hyaluronic acid derivatives have been widely developed to be used as post-operative adhesion-preventing films or gels, materials for wrinkle treatment, materials for plastic surgery, materials for arthritis treatment, vehicles for drug delivery system, etc. Especially, increasing attention has been focused on hyaluronic acid derivative gel, due to peculiar properties thereof, in many application fields. For example, U.S. Patent No. 5,356,883 discloses hyaluronic acid derivative gel in which carboxyl group of hyaluronic acid, or a salt thereof, has been modified to O-acyl or N-acyl ureas by using various kinds of carbodiimides. U.S. Patent No. 5,827,937 discloses a cross-linked polysaccharide gel obtained by cross-linking reaction consisting of two steps. Further, U.S. Patent No. 5,399,351 discloses methods for preparing gels having various properties. SUMMARY OF THE INVENTION One object of the present invention is to provide hyaluronic acid derivative gels in which an amine group-containing saccharide compound is attached to a hyaluronic acid by amidation. Another object of the present invention is to provide hyaluronic acid derivative gels having various different properties to heat, depending upon reaction conditions. A further object of the present invention is to provide a method for preparing hyaluronic acid derivative gels having various different properties by heat treatment Hyaluronic acid derivative gels in accordance with the present invention are prepared by bonding a hyaluronic acid, having a variety of molecular weights, and amine group-containing saccharide compounds, having a variety of molecular weights, via amidation. These hyaluronic acid derivative gels have excellent viscoelastic properties and can thus be applied to many uses. Especially, the hyaluronic acid derivative gels of the present invention are materials showing heat-specific responses and can be made to gels having various different properties by heat treatment Moreover, the present invention provides various hyaluronic acid derivatives having various properties to heat, which can be prepared depending upon the amidation reaction conditions. Additionally, since the hyaluronic acid derivative gels according to the present invention have covalent bonds, i.e., amide bonds, between hyaluronic acid and an amine group-containing saccharide compound, they can withstand several conditions in vivo. These gels are novel biocompatible materials having largely different properties from the existing hyaluronic acid derivatives synthesized using carbodiimide compound. A method for preparing hyaluronic acid derivative gels in accordance with the present invention comprises mixing a solution of hyaluronic acid and a solution of amine group-containing saccharide compound to form ionic bonds between them, then reacting the anionic carboxyl groups of hyaluronic acid with the cationic amine groups of saccharide compound by using an agent for activating carboxyl group, and washing the reactant with water or an acid solution to yield the refined material, followed by separating it and then drying. In other words, the hyaluronic acid derivative gels can be prepared through the procedure comprising a step of mixing/agitating hyaluronic acid and an amine group-containing saccharide compound, a step of activating the carboxyl group of the hyaluronic acid, and a step of reacting the activated carboxyl group of the hyaluronic acid with the amine group of the saccharide compound. The above procedure has advantages that the reaction process is easy, the separation step is simple, and no harmful organic solvents are used. The hyaluronic acid, or its cationic salt, used in the present invention is preferably one or more selected from a group consisting of sodium hyaluronate, potassium hyaluronate, ammonium hyaluronate, calcium hyaluronate, magnesium hyaluronate and tetrabutylammonium hyaluronate. A final reaction concentration of said hyaluronic acid is preferably in the range of between 0.05 mg/ml and 50 mg/ml. A "final reaction concentration," as that term is used herein, of a certain component (A) means a concentration of the component (A) in a total reaction solution also containing other components (B, C...) in addition to the component (A). An average molecular weight of said hyaluronic acid is preferably in the range of between 500,000 and 5,000,000. Said amine group-containing saccharide compound is one or more selected from a group consisting of chitosan, chitosan derivatives, deacetylated hyaluronic acid and deacetylated hyaluronic acid derivatives. Said amine group-containing saccharide compound is preferably added in an amount such that the ratio of the amine group to the carboxyl group of hyaluronic acid is in the range of between 0.01 and 100 (molar equivalents of the amine group to 1 molar equivalent of the carboxyl group). As mentioned earlier, activation of the carboxyl group can be induced using an activating agent The activating agent is not specifically limited as long as it can activate the carboxyl group of hyaluronic acid and is soluble in water, but preferably is a mixture of one or more compounds, as a main agent, selected from a group consisting of l-alkyl-3-(3-dimelhylaminopropyl) carbodiimides (alkyl herein is alkyl of 1-10 carbon atoms), l-ethyl-3-(3-(trimethylammonio)propyl) carbodiimide ("ETC") and l-cyclohexyl-3-(2-morpholinoethyl) carbodiimide ("CMC"), and one or more compounds, as an auxiliary agent, selected from a group consisting of l-hydroxybenzotriazole ("HOBt"), 3,4-dihydro-3-hydroxy-4-oxo-l,2,3-benzotriazine ("HOOBt"), l-hydroxy-7-azabenzotriazole ("HOAt"), iV-hydroxysu