Speedup of initialization for cosmology objects

* Speed up initialization by about a factor of 100 by pre-computing
  some unit conversion quantities.  It turns out this was the
  dominant cost, particularly the call to const.G.cgs.
* import astropy.cosmology time falls by about 50%
* Passes all unit tests
* Closes #2479 (sorta; some more sophisticated options were
  also discussed for that, and this just does the simple bit).
PEP8ify cosmology.core
Fixes for @nhmc comments, more pep8ification.
Merge pull request #2542 from aconley/cosmology_init_speedup

Speedup of initialization for cosmology objects