Add the ability to use and interpret Galsim galaxy catalogues

[york-stsci]

Galsim galaxies can provide much more realistic background galaxies than the BC95 catalogue, and I've been learning about how to use galsim recently, so this should be both useful and straightforward. It will potentially require more modules to be added, and in addition may require more data files to actually use galsim.

The galsim catalogue should be optional (i.e. galsim should only be imported, and its data files required, if STIPS actually runs into a galsim catalogue).

[york-stsci]

In 2.0.0, STIPS now has the ability (if GalSim is available) to model galaxies using GalSim. It does not currently include the ability to use galsim-style catalogues.