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).