VEHICLE HEADLINER COMPRISED OF A THERMOFORMABLE THERMOPLASTIC FOAM SHEET Background of the Invention The present invention relates to a vehicle headliner having a thermoformed core layer comprised of an extruded thermoplastic foam. The core layer and the headliner are capable of substantially maintaining their shape and contour when installed in a vehicle even at elevated temperatures. Headliners are laminates that are applied to the undersides of roofs of passenger compartments in vehicles. Headliners serve a variety of purposes, including cushioning, aesthetics, thermal insulation, and sound insulation. Headliners employed in commercially produced vehicles are relatively complex and highly engineered due to the physical demands and environmental conditions to which headliners are exposed. The headliner must have sufficient rigidity to prevent sagging due to gravity yet be pliable enough to permit fabrication and/or thermoforming and installation. The headliner may also be a component of an overall impact protection system that provides some degree of cushioning in the event of sudden contact by an occupant in the passenger compartment. The headliner must also be able to withstand elevated temperatures associated with exposure of the vehicle to the solar heating. The headliner must also be able to be formed into a desired shape, configuration, or contour. Headliners currently employed commercially typically are formed with multiple layers of polyurethane foam and glass/fiberglass mattes or scrim adhered with polyurethane adhesives and pressed together under heat to a desired shape and contour. Felt or fabric is typically applied to the headliner at the surface that is to face the interior of the passenger compartment. Representative headliners include those disclosed in U.S. Patent Nos. 5,460,870; 5,486,256; 5,582,906; and 5,670,211, which are incorporated herein by reference. The disadvantages of current commercial headliners are many. They are costly due to expensive component materials and complicated manufacturing methods and processes. Thermoset adhesives such as polyurethane adhesives are expensive and require multi-step, time-consuming application and curing methods and processes. Reinforcing fabric or scrim of glass, fiberglass, carbon, or other materials in fibrous form as well as polyurethane foam sheets must also be interspersed in a layered array along with the thermoset adhesives. These reinforcing materials are expensive and require multi-step integration into the manufacturing product along with the thermoset adhesives. Further, the component materials and the finished headliner product are usually difficult if not impossible to efficiently recycle. It would be desirable to have a headliner which is comprised of less expensive component materials and which can be assembled by a manufacturing process which is less complex and expensive. It would further be desirable to have a headliner that offers performance features and attributes similar to those afforded by headliners employing thermoset materials and reinforcing fabric or scrim. It would further be desirable if the headliner and/or material components thereof would be readily recyclable. Summary of the Invention According to the present invention, there is a vehicle having an improved headliner. The vehicle has a passenger compartment therein and a roof situated overhead of the passenger compartment. The headliner is situated adjacent the underside of the roof. The headliner comprises a thermoformed core layer. The core layer comprises an extruded thermoplastic foam and optionally one or more adjacent layers of a substantially non-foamed thermoplastic. The core layer is substantially free of thermoset materials and preferably free of glass/fiberglass mattes or scrim. The core layer is substantially resistant to sagging and able to substantially maintain its thermoformed shape when the headliner is installed in the vehicle. A decorative layer such a felt layer or a fabric layer is preferably laminated to the core layer. Further according to the invention, there is a process for assembling or making a headliner and installing it in a vehicle. The process comprises: a) providing a thermoformable core layer comprising an extruded, thermoplastic foam and optionally one or more adjacent layers of a substantially non-foamed thermoplastic wherein the core layer is substantially free of thermoset materials, substantially resistant to sagging, and able to substantially maintain its shape when the headliner is installed in the vehicle; b) thermoforming the core layer by applying heat and mechanical pressure thereto to form the headliner; c) installing or applying the headliner adjacent the underside of the roof. Preferably, a decorative layer is laminated to the surface of the core layer that is to face the interior of the passenger compartment. The decorative layer may be laminated to the core layer either prior to or after the thermoforming of the core layer. Brief Description of the Drawings Figure 1 is a fragmentary, perspective view partly in cross-section of a vehicle having a headliner. Figure 2 is a side view of the cutaway of Figure 1 wherein the roof and headliner of the vehicle are shown. Figure 3 is a fragmentary, perspective view of a roof of a vehicle having a headliner wherein the headliner is shown in cutaway-Figure 4 is a fragmentary, perspective view of a roof of a vehicle having a headliner wherein the headliner is shown in cutaway. Figure 5 shows a portion of a headliner having foamed portion and non-foamed portions. Figures 6-11 illustrate several configurations of a portion of a headliner assembly. Detailed Description of the Invention The headliner affords many advantages not known in the art with respect to a single headliner. Those advantages are largely related to the thermoformable core layer present in the headliner. Those advantages include the following: the headliner is readily thermoformable to a desired shape, configuration, or contour; the headliner is rigid enough to prevent sagging and substantially maintain its shape - even when exposed to elevated temperatures commonly encountered in vehicles in hot weather and/or direct sunlight conditions; the headliner is preferably comprised of relatively inexpensive and recyclable thermoplastics. The headliner offers good cushioning properties against head bumps and exhibits good sound and thermal absorption. The core layer comprising the thermoplastic foam in the present invention affords significant advantages versus other foam layers taught in the prior art as useful in headliners. U.S. Patent Nos. 5,670,211 discloses headliners having flexible or semirigid foam sheets of polyurethane foam which are processed and formed into a headliner with polyurethane adhesives. The patent also discloses that other foam sheets of PPO, expanded polystyrene, and expanded polypropylene may be substituted for the polyurethane foam. The core layer in the present invention is advantageous over the teachings of the patent in that it is substantially free of thermoset adhesives and can be more easily recycled. The core layer comprising the extruded thermoplastic foam is also more readily thermoformable than polyurethane foam and typically stronger for a given density than expanded (bead) foams, and exhibits higher heat distortion resistance than polystyrene foams. U.S. Patent No. 3,637,458 discloses very thin, extruded polypropylene foam sheets described as useful in a laundry list of applications, including as a headliner. The patent offers no teaching as to the structure of such a headliner or the function that the disclosed polypropylene foam sheet has in it. Foams can serve different functions in a headliner such as to provide structure and mechanical performance or as a cushion backing for a decorative fabric layer. U.S. Patent No. 5,536,793 discloses a polyester foam described as useful in many applications, including as a headiiner. The teachings of this patent are deficient for the same reasons as those of U.S. Patent No. 3,637,458. The figures depict embodiments of the present invention, in Figure 1, vehicle 10 has a roof 12, a passenger compartment 14, and a headiiner 16. Figure 1 shows a cutaway along a circular dotted line 2-2, which corresponds to the cross-sectional side view seen in Figure 2. Figure 2 shows roof 12 having a headiiner 16 adhered or affixed thereto. Headiiner 16 comprises a conventional extruded thermoplastic foam layer 20, an adhesive layer 22, and a fabric layer 24. Figure 3 shows another embodiment of a roof/headliner combination. In Figure 3, there is a headiiner 30 adhered or affixed to a vehicle roof 32. Headiiner 30 comprises an extruded, coalesced strand thermoplastic foam 34 and a fabric layer 36. In headiiner 30, the strands are oriented generally vertically and generally perpendicular to the plane of roof 32. Figure 4 shows another embodiment of a roof/headliner combination. In Figure 4, there is a headiiner 40 adhered or affixed to a vehicle roof 42. Headiiner 40 comprises an extruded, coalesced strand thermoplastic foam 44 and a fabric layer 46. In headiiner 40, the strands are oriented generally horizontally and generally parallel to the plane of roof 42. Adhesives known in the art may be employed to adhere various layers of the headiiner to each other or the headiiner to the roof of the car. Useful adhesives include thermoset adhesives such as polyurethane resins and epoxies and thermoplastic adhesives such as polyethylenes, polypropylenes, ethylene copolymers; propylene copolymers; and the like. Useful adhesives are taught in U.S. Patent Nos. 5,460,870 and 5,670,211. The adhesives may be applied by any means known in the art such as by spraying, coating, or in film form. Preferred adhesives are thermoplastic because of their lower cost and potential recyclability. The presence of an adhesive is not critical to the present invention. The foam may be closed cell or open cell Open cell content is determined according to ASTM D2856-A. Closed cell foams provide advantages of better thermal insulating capability and resiliency and open cell foams provide advantages of better sound insulation, dimensional stability, and heat transfer during thermoforming. The thermoplastic foam preferably has a density before thermoforming of from about 16 to about 200 and more preferably from about 16 to about 80 kilograms per cubic meter. The foam has an average cell size of preferably from about 0.1 to about 5.0 and preferably from about 0.2 to about 3.0 millimeters according to ASTM D3576. The indicated foam density and cell size ranges are general in nature for thermoplastic foams. Most preferred density and cell size ranges will vary depending upon the composition of the foam and desired physical properties. For instance, a foam can usually be made more rigid by increasing density or cell size. Particularly desirable foams are those of propylene polymers, polyesters, and polyamides having a density before thermoforming of from about 16 to about 160 kilograms per cubic meter and preferably about 24 to about 100 kilograms per cubic meter. The thermoplastic foam is preferably extruded as a unitary structure but may also be formed by laminating together two or more relatively thinner thermoplastic foam sheets by any means known in the art such as thermal welding or adhesive layers. The foam must be resistant to heat distortion and be dimensionally stable at elevated temperatures commonly encountered at the roof of a vehicle due to solar heating. The foam preferably exhibits a dimensional stability of about 5 percent or less and more preferably about 1 percent or less with respect to both expansion and shrinkage according to SAE 883. The foams may be made in any cross-sectional size or configuration such as foam sheet or plank. Particularly useful foams are those having a minor dimension in cross-section (thickness) of 1.5 millimeters or more and preferably 3 millimeters or more. A layer or layers of a decorative material such a felt or fabric may be applied to the surface of the headliner facing the interior of the passenger compartment or interior cabin for aesthetic appeal. The layer may be of any type known in the art. Those most typically employed commercially are felts or woven fabrics. Useful fabrics include those of woven polyester, nylon, and polypropylene fibers. Preferably, the felt or fabric layer is comprised of the same or similar polymeric material as the foam. The felt or fabric layer may be adhered to the foam by any means known in the art such as thermal welding, adhesive films, or adhesive liquids or coatings. A preferred decorative layer is a woven fabric of thermoplastic fibers thermally welded to the core layer without the benefit of adhesives. Thermal welding refers to the heating of the fabric layer to an extent such that the fibers become tacky or sticky and are able to adhere to the core layer without the benefit of an adhesive. A fabric layer may be thermally welded to a core layer if applied to the core layer during thermoforming or when the core layer is otherwise at an elevated temperature. A preferred headliner is comprised entirely of recyclable materials. Useful recyclable materials include propylene polymers such as polypropylene; high-density polyethylene; polyesters such as polyethylene terephthalate; and polycarbonates. A most preferred headliner is comprised entirely of recyclable materials of similar composition such that they can be recycled together. For instance, a headliner may comprise any of the following: a laminate of a propylene polymer foam and a woven polypropylene fabric layer; a laminate of a polyethylene terephthalate foam and a woven polyethylene terephthalate fabric layer; or a polyamide (nylon) foam and a polyamide fabric layer. If desired, different recyclable materials may be employed together such as the following: a) a laminate of a propylene polymer foam and a woven fabric layer of a polyester or a polyamide and b) a laminate of a polyester foam and a woven fabric layer of polypropylene or a polyamide. The foam is readily thermoformable to a desired shape, configuration, or contour. Typically, the foam and the remainder of the headliner are of substantially the same shape, configuration, or contour as the roof of the vehicle since the headliner is positioned underneath the roof. The term "thermoformable" means the foam may be thermoformed or otherwise shaped under heat and mechanical pressure by any conventional means known in the art to a different shape or contour. Typically, the foam is provided in the form of a substantially flat sheet or plank and pressed under heat and pressure to form a contoured sheet similar in shape and contour to the roof of the vehicle under which it is to be positioned. If desired, a decorative layer such as a fabric layer of woven thermoplastic fibers may be thermally welded to the foam during the thermoforming process. The physical properties and thermal resistance of the foam may be enhanced by forming or inducing the formation of a substantially non-foamed skin on the foam such as by laminating plastic films or sheets to the foam, by coating it with a plastic resin, heating a surface or surfaces of the foam above its glass transition temperature or the melting point to collapse the cellular structure at the skin, or a combination of any of the foregoing. The film, sheet, or coating may comprise any known thermoplastic resin orthermoset resin. Useful thermoplastic resins include those described above with respect to those composing the foam and useful thermoset resins include polyurethanes and epoxies. The headliner may be applied to the underside of the roof of the vehicle by any means known in the art such as by adhesion or by affixing by mechanical means. Mechanical means include dips, side molding, and overhead (dome) light assemblies. Useful thermoplastic foams include but are not limited to those of propylene polymer, polyester, polyamide, polycarbonate, high density polyethylene, chlorinated polyethylene, polyphenylene oxides, blends of polyphenylene oxides and polystyrene, propylene/ethylene copolymers, thermoplastic polyurethanes, blends of EPDM and polyethylene, blends of polypropylene and EPDM, blends of polypropylene and ethylene/styrene copolymers. Ethylene/styrene copolymers and foams containing same are taught in U.S. Patent No. 5,460,818, which is incorporated herein by reference. Preferred foams include propylene polymer foams and polyester foams. More preferred foams include polypropylene foams, foams of propylene/ethylene copolymers of 95/5 to 99.5/0.5 monomeric weight ratio, and polyethylene terephthalate foams. A most preferred foam for the core layer is an extruded propylene polymer foam. Suitable propylene polymer materials include propylene homopolymers (polypropylene) and copolymers of propylene and copolymerizable ethylenically unsaturated comonomers. The propylene polymer material may further include non-propylenic polymers. The propylene polymer material may be comprised solely of one or more propylene homopolymers, one or more propylene copolymers, a blend of one or more of each of propylene homopolymers and copolymers, or blends of any of the foregoing with a non-propylenic polymer. Regardless of composition, the propylene polymer material comprises at greater than 50 and preferably about 70 weight percent or more of propylene monomeric units. Suitable monoethylenicalty unsaturated comonomers include olefins, vinylacetate, methylacrylate, ethylacryiate, methyl methacryiate, acrylic acid, itaconic acid, maleic acid, maleic anhydride, and the like. The propylene copolymer preferably comprises about 45 percent or less by weight of the ethylenically unsaturated comonomer. Suitable non-propylenic polymers incorporatable in the propylene polymer material include high, medium, low, and linear density poiyethyienes, polybutene-1, ethylene/acrylic acid copolymer, ethylene/vinyl acetate copolymer, ethylene/propylene copolymer, styrene/butadiene copolymer, ethylene/styrene copolymer, ethylene/ethyl acrylate copolymer, ionomer and the like. Particularly useful propylene copolymers are those copolymers of propylene and one or more non-propylenic olefins. Propylene copolymers include random, block, and grafted copolymers of propylene and an olefin selected from the group consisting of ethylene, C4- C-jQ 1-olefins, and C4-C10 dienes. Propylene copolymers also include random terpoiymers of propylene and 1-olefins selected from the group consisting of ethylene and C4-C8 1- olefins. In terpoiymers having both ethylene and C4-C8 1-olefins, the ethylene content is preferably 45 percent or less by weight. The C4-C10 1-olefins include the linear and branched C4-C10 1-olefins such as, for example, 1-butene, isobutylene, 1-pentene, 3- methyl-1-butene, 1-hexene, 3,4-dimethyl-1-butene, 1-heptene, 3-methyl-1-hexene, and the like. Examples of C4-C10 dienes include 1,3-butadiene, 1,4-pentadiene, isoprene, 1,5- hexadiene, 2,3-dimethyl-1,3-hexadiene, and the like. Also, as used herein, the propylene polymer material has a melt flow rate of between about 0.05 and 50 and preferably between 0.1 and 20 according to ASTM D1238 Condition L. Preferred propylene polymer resins are those propylene resins that are branched or lightly cross-linked polymer materials. Branching (or light cross-linking) may be obtained by those methods generally known in the art, such as chemical or irradiation branching/light cross-linking. One such resin which is prepared as a branched/lightly cross-linked polypropylene resin prior to using the polypropylene resin to prepare a finished polypropylene resin product and the method of preparing such a polypropylene resin is described in U.S. Patent No. 4,916,198, which is hereby incorporated by reference. Another method to prepare branched/lightly cross-linked polypropylene resin is to introduce chemical compounds into the extruder, along with a polypropylene resin and allow the branching/lightly cross-linking reaction to take place in the extruder. U.S. Patent No. 4,714,716 illustrates this method and is incorporated by reference. Useful extruded propylene polymer foams are seen in U.S. Patent Nos. 5,348,795; 5,527,573; and 5,567,742, which are incorporated herein by reference. Useful extruded polyester foams, including polyethylene terephthalate (PET) foams, are seen in U.S. Patent Nos. 5,000,991; 5,234,640; and 5,536,793, which are incorporated herein by reference. Foams can be made from other useful thermoplastics such as high-density polyethylene, chlorinated polyethylene, TPO mixtures of EPDM rubbers(ethylene/propylene/diamine copolymers)and polyethylene. Thermoplastic foams useful are preferably non-crosslinked, but may be lightly crosslinked. The term "non-crosslinked" is inclusive however, of the slight degree of crosslinking that occurs naturally without the use of crosslinking agents or radiation. Non-crosslinked foams contain less than 5 percent gel per ASTM D2765-84, Method A. Lightly crosslinked foams contain 5-15 percent gel per ASTM D2765-84, Method A. The blowing agent may comprise any known in the art such as chemical blowing agents and physical blowing agents of organic and/or inorganic composition. Suitable inorganic blowing agents useful include carbon dioxide, nitrogen, argon, water, air, nitrogen, and helium. Suitable organic blowing agents include aliphatic hydrocarbons having 1-9 carbon atoms and halogenated aliphatic hydrocarbons, having 1-4 carbon atoms. Aliphatic hydrocarbons include methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane, and the like. Also included are the alcohols such as ethanol, methanol, and propanol. Among halogenated hydrocarbons, fluorinated hydrocarbons are preferred. Examples of fluorinated hydrocarbon include methyl fluoride, perfluoromethane, ethyl fluoride, 1,1-difiuoroethane, 1,1,1-trifluoroethane (HFC-143a), 1,1,1,2-tetrafluoro-ethane (HFC-134a), pentafluoroethane, perfluoroethane, 2,2-difluoropropane, 1,1,1- trifluoropropane, perfluoropropane, perfluorobutane, perfiuorocyclobutane. Partially halogenated chlorocarbons and chlorofluorocarbons for use in this invention include methyl chloride, methylene chloride, ethyl chloride, 1,1,1-trichloroethane, 1,1-dichloro-1-fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b), 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1