Rather than fall back on built-in cosmology, just require astropy.cosmology

* Careful comparison shows that figure 8.3 (rbf_ridge) does not
  quite match that in the text in the left panel.  However, that
  is not the result of using astropy.cosmology, but rather seems to
  be some difference in scikit-learn.  Comparison with the master
  branch (so, using the built-in Cosmology class) produces an
  identical figure to the one produced by this branch.
* Change deprecation warning to recommend using astropy.cosmology
* Add optional arguments to generate_mu_z and fetch_great_wall
  to allow callers to provide their own cosmology, while keeping
  the same defaults.