Moved mujoco converting stuff to VR/XML/mjcf. Began integration to RobotIO

VirtualRobot/CMakeLists.txt

VirtualRobot/XML/RobotIO.cpp

VirtualRobot/XML/RobotIO.h

VirtualRobot/XML/mjcf/MasslessBodySanitizer.cpp

VirtualRobot/XML/mjcf/MasslessBodySanitizer.h

VirtualRobot/XML/mjcf/MjcfDocument.cpp

VirtualRobot/XML/mjcf/MjcfDocument.h

VirtualRobot/XML/mjcf/MujocoIO.cpp

VirtualRobot/XML/mjcf/MujocoIO.h

VirtualRobot/XML/mjcf/exceptions.cpp

VirtualRobot/XML/mjcf/exceptions.h

VirtualRobot/XML/mjcf/utils.cpp

VirtualRobot/XML/mjcf/utils.h

VirtualRobot/XML/mjcf/xml_visitors.cpp

VirtualRobot/XML/mjcf/xml_visitors.h

VirtualRobot/examples/MjcfConverter/CMakeLists.txt

VirtualRobot/examples/MjcfConverter/main.cpp