code: def mass_3d(self, r, rho0, Ra, Rs):
          
          m_3d = 4 * np.pi * rho0 * Ra ** 2 * Rs ** 2 / (Rs ** 2 - Ra ** 2) * (Rs * np.arctan(r / Rs) - Ra * np.arctan(r / Ra))
          return m_3d
docs: mass enclosed a 3d sphere or radius r
          :param r:
          :param Ra:
          :param Rs:
          :return: