Add mj_copyBack for copying real-valued arrays from mjModel back to mjSpec.

Also add `SaveAndReadXML` function to test fixture using `mjSpec` as input.

PiperOrigin-RevId: 765393821
Change-Id: Ic257addd2fc89678fc11b226c168c077626cc51e

src/user/user_api.cc

@@ -375,6 +375,14 @@ int mjs_setDeepCopy(mjSpec* s, int deepcopy) {
 
 
 
+// copy real-valued arrays from model to spec, returns 1 on success
+int mj_copyBack(mjSpec* s, const mjModel* m) {
+  mjCModel* model = static_cast<mjCModel*>(s->element);
+  return model->CopyBack(m);
+}
+
+
+
 // delete object, return 0 on success
 int mjs_delete(mjsElement* element) {
   mjCModel* model;