Update default cosmology

[jesford]

Currently the default cosmology for ClusterEnsemble() is astropy.cosmology.Planck13, but we should eventually update to astropy.cosmology.Planck15, or newer if something else comes out.

Note that this will cause many tests to fail, because they assume Planck13. Will need to either just adapt calls to ClusterEnsemble() to specify the previously-default cosmology, or add tests for the other cosmology case (the latter will require more work, but is obviously the better option).