getGravCompEfforts calculates joint efforts that compensate
  for gravity.
 
    This method computes the efforts that are required to
    cancel out the forces on an arm caused by gravity
 
    'Positions' argument expects a [1 x numDoF] vector of
    positions of all degrees of freedom.
 
    'GravityVector' argument expects an [3 x 1] vector of the
    direction of gravity in the base frame.  Note that this
    direction vector is not required to be unit length, and the
    magnitude of gravitational acceleration is always assumed
    to be 9.81 m/s^2.
 
    Example
       % Compensate gravity at current feedback positions
       fbk = group.getNextFeedback();
       gravity = [0 0 -1];
       efforts = kin.getGravCompEfforts(fbk.position, gravity);