Moved Attribute, Element and others to elements/core

VirtualRobot/CMakeLists.txt

VirtualRobot/MJCF/elements/asset.h

VirtualRobot/MJCF/elements/body.h

VirtualRobot/MJCF/elements/compiler.h

VirtualRobot/MJCF/elements/contact.h

VirtualRobot/MJCF/elements/core/Attribute.cpp

VirtualRobot/MJCF/elements/core/Attribute.h

VirtualRobot/MJCF/elements/core/Element.cpp

VirtualRobot/MJCF/elements/core/Element.h

VirtualRobot/MJCF/elements/core/exceptions.cpp

VirtualRobot/MJCF/elements/core/exceptions.h

VirtualRobot/MJCF/elements/core/mjcf_utils.cpp

VirtualRobot/MJCF/elements/core/mjcf_utils.h

VirtualRobot/MJCF/elements/custom.h

VirtualRobot/MJCF/elements/default.h

VirtualRobot/MJCF/elements/equality.h

VirtualRobot/MJCF/elements/keyframe.h

VirtualRobot/MJCF/elements/option.h

VirtualRobot/MJCF/elements/sensor.h

VirtualRobot/MJCF/elements/size.h