computations seem to work in recursions of DFSPolicy, but we should find a better design, since it is to cumbersome to specifiy all the states and operation types
isValid methods that check for validity of joint and chain at construction time has been implemented and tested. For joints it makes sure that the frames used to construct a joint are of JointFrameType and do not belong to the same segment. For the kinematic chain it makes sure that no joint shares the JointFrames of another joint, thus avoiding that several joints are collacated.