Add mjs_attachToSite for attaching a body to a site.

PiperOrigin-RevId: 684432183
Change-Id: I7100bebfb6521f9dbfbb07f31490b2b9298bf18c

src/user/user_objects.h

@@ -587,6 +587,9 @@ class mjCSite : public mjCSite_, private mjsSite {
 
   mjsSite spec;                   // variables set by user
 
+  // site's body
+  mjCBody* Body() const { return body; }
+
   // use strings from mjCBase rather than mjStrings from mjsSite
   using mjCBase::name;
   using mjCBase::info;