Add copy constructor and assignment to mjCModel.

PiperOrigin-RevId: 616850780
Change-Id: I7fedab2efa2a90fd7ce952eac4e195f77c5ff5bc

src/user/user_api.h

@@ -731,6 +731,9 @@ typedef struct _mjmDefault {       // default specification
 // Create model.
 MJAPI mjSpec* mjm_createSpec();
 
+// Copy model.
+MJAPI mjSpec* mjm_copySpec(const mjSpec* s);
+
 // Copy back model.
 MJAPI void mjm_copyBack(mjSpec* s, const mjModel* m);