Moved element types into types/

VirtualRobot/CMakeLists.txt

VirtualRobot/MJCF/elements.h

VirtualRobot/MJCF/elements/types/actuator.cpp

VirtualRobot/MJCF/elements/types/actuator.h

VirtualRobot/MJCF/elements/types/asset.cpp

VirtualRobot/MJCF/elements/types/asset.h

VirtualRobot/MJCF/elements/types/body.cpp

VirtualRobot/MJCF/elements/types/body.h

VirtualRobot/MJCF/elements/types/compiler.cpp

VirtualRobot/MJCF/elements/types/compiler.h

VirtualRobot/MJCF/elements/types/contact.cpp

VirtualRobot/MJCF/elements/types/contact.h

VirtualRobot/MJCF/elements/types/custom.cpp

VirtualRobot/MJCF/elements/types/custom.h

VirtualRobot/MJCF/elements/types/default.cpp

VirtualRobot/MJCF/elements/types/default.h

VirtualRobot/MJCF/elements/types/equality.cpp

VirtualRobot/MJCF/elements/types/equality.h

VirtualRobot/MJCF/elements/types/keyframe.cpp

VirtualRobot/MJCF/elements/types/keyframe.h