code: def c_rho0(self, rho0):
          
          if not hasattr(self, '_c_rho0_interp'):
              c_array = np.linspace(0.1, 10, 100)
              rho0_array = self.rho0_c(c_array)
              from scipy import interpolate
              self._c_rho0_interp = interpolate.InterpolatedUnivariateSpline(rho0_array, c_array, w=None, bbox=[None, None], k=3)
          return self._c_rho0_interp(rho0)
docs: computes the concentration given a comoving overdensity rho0 (inverse of function rho0_c)
          :param rho0: density normalization in h^2/Mpc^3 (comoving)
          :return: concentration parameter c