Move mjCSite attributes to mjmSite struct.

PiperOrigin-RevId: 603051589
Change-Id: I31eeeccffa15b96e7393b8e64f6be8c964fc5948

src/user/user_model.h

@@ -18,6 +18,7 @@
 #include <functional>
 #include <map>
 #include <string>
+#include <string_view>
 #include <utility>
 #include <vector>
 
@@ -192,6 +193,9 @@ class mjCModel {
   template<class T>              // if asset name is missing, set to filename
   void SetDefaultNames(std::vector<T*>& assets);
 
+  template <class T>             // delete material from object
+  void DeleteMaterial(std::vector<T*>& list, std::string_view name = "");
+
   //------------------------ compile phases
   void MakeLists(mjCBody* body);        // make lists of bodies, geoms, joints, sites
   void IndexAssets(bool discard);       // convert asset names into indices