Make default class members private.

PiperOrigin-RevId: 634288934
Change-Id: I1b7a5ec8dd51c65377b09bcb85c56a849573cac7

src/user/user_model.h

@@ -222,7 +222,8 @@ class mjCModel : public mjCModel_, private mjSpec {
   std::string get_meshdir() const { return meshdir_; }
   std::string get_texturedir() const { return texturedir_; }
 
-  const std::vector<mjCDef*>& Defaults() const { return defaults_; }
+  mjCDef* Defaults(int i) const { return defaults_[i]; }
+  int NumDefaults() const { return defaults_.size(); }
 
   const std::vector<std::pair<const mjpPlugin*, int>>& ActivePlugins() const {
     return active_plugins_;