code: def mass_in_theta_E(self, theta_E):
          
          mass = self.arcsec2phys_lens(theta_E) ** 2 * np.pi * self.epsilon_crit
          return mass
docs: mass within Einstein radius (area * epsilon crit) [M_sun]
          :param theta_E: Einstein radius [arcsec]
          :return: mass within Einstein radius [M_sun]