More cosmology options

clusters.ClusterEnsemble() accepts input cosmology as any astropy.cosmology object

Python 3 compatibility

• Confirmed to run on Python 3.4 and 3.5, as well as 2.7.
• Minor API change, where I had misspelled "richness" in function names.
  • mass_to_richess() => mass_to_richness()
  • richess_to_mass() => richness_to_mass()

Initial release

v0.1.0

first release