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_urdf.h

@@ -34,9 +34,12 @@ class mjXURDF : public mjXBase {
   mjXURDF();                                          // constructor
   virtual ~mjXURDF();                                 // destructor
 
+  // parse and set frame of base link and append a prefix to the name
+  void Parse(tinyxml2::XMLElement* root, const std::string& prefix, double* pos, double* quat);                           // parse and set frame of base link
   void Parse(tinyxml2::XMLElement* root);             // main parser
 
  private:
+  std::string GetPrefixedName(const std::string& name);            // get prefix/name of element
   int FindName(std::string name, std::vector<std::string>& list);  // find name in list
   void AddName(std::string name, std::vector<std::string>& list);  // add name to list
   void AddBody(std::string name);                                  // add body to local table
@@ -57,6 +60,8 @@ class mjXURDF : public mjXBase {
   std::vector<std::string> urMat;               // material name
   std::vector<mjRGBA> urRGBA;                   // material RBG value
   std::unordered_set<std::string> urGeomNames;  // geom name
+
+  std::string urPrefix;                         // prefix to apply to all names
 };
 
 #endif  // THIRD_PARTY_MUJOCO_SRC_XML_XML_URDF_