def project( target_point )
      Util3D.check_arg_type(::Vector3, target_point)
      distance, closest_point = self.distance( target_point )
      return closest_point
    end
def project( target_point )
      Util3D.check_arg_type(::Vector3, target_point)
      distance, closest_point = self.distance( target_point )
      return closest_point
    end
[Input]
  _target_point_ should be Vector3.
 [Output]
  retrun projected point on plane as Vector3.