code: def inverse_jacobian(self, maps):
          
          m1 = maps[parameters.mass1]
          m2 = maps[parameters.mass2]
          mchirp = conversions.mchirp_from_mass1_mass2(m1, m2)
          eta = conversions.eta_from_mass1_mass2(m1, m2)
          return -1. * mchirp / eta**(6./5)
docs: Returns the Jacobian for transforming mass1 and mass2 to
          mchirp and eta.