Initial Implementation of MuJoCo Physics Device

1) Modifications to MJCModel and URDF parsing to enable loading multiple URDF's
2) Device Skeleton for MuJoCo Physics Device
3) Scene Loading and Modifications
4) State Generation

PiperOrigin-RevId: 539676439
Change-Id: Ic65f812b660319443e74f68acd851dba1435ddca

src/xml/xml.cc

@@ -333,6 +333,12 @@ mjCModel* mjParseXML(const char* filename, int vfs_provider, char* error, int er
     else if (!strcasecmp(root->Value(), "robot")) {
       // parse URDF model
       mjXURDF parser;
+
+      // set reasonable default for parsing a URDF
+      // this is separate from the Parser to allow multiple URDFs to be loaded.
+      model->strippath = true;
+      model->fusestatic = true;
+
       parser.SetModel(model);
       parser.Parse(root);
     }