function getDual(out, a) {
    out[0] = a[4];
    out[1] = a[5];
    out[2] = a[6];
    out[3] = a[7];
    return out;
  }
Gets the dual part of a dual quat
@param  {quat} out dual part
@param  {ReadonlyQuat2} a Dual Quaternion
@return {quat} dual part