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.h

@@ -92,7 +92,7 @@ class mjXBase : public mjXUtil {
   };
 
   // set the model allocated externally
-  virtual void SetModel(const mjSpec*, const mjModel* = nullptr);
+  virtual void SetModel(mjSpec*, const mjModel* = nullptr);
 
   // read alternative orientation specification
   static int ReadAlternative(tinyxml2::XMLElement* elem, mjsOrientation& alt);