add mjs_detachDefault to delete a default class from mjSpec

PiperOrigin-RevId: 738452369
Change-Id: I5a53a6dc8f89e1e8136aeefc09d7d9b7b7204e1f

src/user/user_objects.cc

@@ -536,12 +536,16 @@ int mjCBoundingVolumeHierarchy::MakeBVH(
 
 //------------------------- class mjCDef implementation --------------------------------------------
 
+
+
 // constructor
 mjCDef::mjCDef() {
   name.clear();
   id = 0;
   parent = nullptr;
+  model = 0;
   child.clear();
+  elemtype = mjOBJ_DEFAULT;
   mjs_defaultJoint(&joint_.spec);
   mjs_defaultGeom(&geom_.spec);
   mjs_defaultSite(&site_.spec);
@@ -561,6 +565,13 @@ mjCDef::mjCDef() {
 
 
 
+// constructor with model
+mjCDef::mjCDef(mjCModel* _model) : mjCDef() {
+  model = _model;
+}
+
+
+
 // copy constructor
 mjCDef::mjCDef(const mjCDef& other) {
   *this = other;