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/xml/xml_base.cc

@@ -44,8 +44,8 @@ mjXBase::mjXBase() {
 
 
 // set model field
-void mjXBase::SetModel(const mjSpec* _model, const mjModel* m) {
-  spec = (mjSpec*)_model;
+void mjXBase::SetModel(mjSpec* _model, const mjModel* m) {
+  spec = _model;
 }