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