Remove mjCBase::def and use mjCBase::classname instead.

PiperOrigin-RevId: 650629741
Change-Id: I10a9955b99e4a5ff0b5a742e22635ccad1455f1a

src/user/user_model.h

@@ -269,6 +269,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   void SaveState(const mjData* d);
   void RestoreState(const mjModel* m, mjData** dest);
 
+  // map from default class name to default class pointer
+  std::map<std::string, mjCDef*> def_map;
+
  private:
   // settings for each defaults class
   std::vector<mjCDef*> defaults_;