function getAngle(a, b) {
    var dotproduct = dot$2(a, b);
    return Math.acos(2 * dotproduct * dotproduct - 1);
  }
Gets the angular distance between two unit quaternions

@param  {ReadonlyQuat} a     Origin unit quaternion
@param  {ReadonlyQuat} b     Destination unit quaternion
@return {Number}     Angle, in radians, between the two quaternions