Convert mjElement to struct rather than opaque pointer.

By making mjElement the parent of mjCBase, mjCModel, and mjCDef, this CL enables safer casting using static_cast instead of reinterpret_cast and C-style casting.

PiperOrigin-RevId: 619526291
Change-Id: Ifa2b4bd3ecd95bade61f41a7005c62212f456c00

src/user/user_model.h

@@ -33,7 +33,7 @@
 typedef std::map<std::string, int, std::less<> > mjKeyMap;
 typedef std::array<mjKeyMap, mjNOBJECT> mjListKeyMap;
 
-class mjCModel_ {
+class mjCModel_ : public mjElement {
  public:
   // attach namespaces
   std::string prefix;