Add childclass support to frames.

PiperOrigin-RevId: 615487831
Change-Id: I0e4d2efd52211701b4286a0b0de84424470a3a47

src/user/user_model.h

@@ -250,6 +250,7 @@ class mjCModel : private mjSpec {
   std::vector<mjCSite*>   sites;    // list of sites attached to this body
   std::vector<mjCCamera*> cameras;  // list of cameras
   std::vector<mjCLight*>  lights;   // list of lights
+  std::vector<mjCFrame*>  frames;   // list of frames
 
   // array of pointers to each object list (enumerated by type)
   std::array<std::vector<mjCBase*>*, mjNOBJECT> object_lists;