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_objects.h

@@ -167,7 +167,7 @@ class mjCBoundingVolumeHierarchy : public mjCBoundingVolumeHierarchy_ {
 //------------------------- class mjCBase ----------------------------------------------------------
 // Generic functionality for all derived classes
 
-class mjCBase_ {
+class mjCBase_ : public mjElement {
  public:
   int id;                 // object id
   std::string name;       // object name
@@ -1606,7 +1606,7 @@ class mjCKey : public mjCKey_, private mjsKey {
 //------------------------- class mjCDef -----------------------------------------------------------
 // Describes one set of defaults
 
-class mjCDef {
+class mjCDef : public mjElement {
   friend class mjXWriter;
 
  public: