'Absorption distance at redshift ``z``.
This is used to calculate the number of objects with some
cross section of absorption and number density intersecting a
sightline per unit redshift path.
Parameters
z : array-like
Input redshifts.  Must be 1D or scalar.
Returns
d : float or ndarray
Absorption distance (dimensionless) at each input redshift.
References
Hogg 1999 Section 11. (astro-ph/9905116)
Bahcall, John N. and Peebles, P.J.E. 1969, ApJ, 156L, 7B'

def absorption_distance(self, z):

    from scipy.integrate import quad
    f = (lambda red: quad(self._abs_distance_integrand_scalar, 0, red)[0])
    return vectorize_if_needed(f, z)
