code: def mass_2d(self, r, rho0, Rs):
          
  
          sigma0 = self.rho2sigma(rho0, Rs)
          return self.mass_2d_lens(r, sigma0, Rs)
docs: mass enclosed projected 2d sphere of radius r
          :param r:
          :param rho0:
          :param a:
          :param s:
          :return: