#343 Test of accuracy of Galsim draw function.

This branch was where Tomek did most of his work checking the accuracy of GalSim's renderings, especially comparing the shapes of galaxies when drawn with photon shooting vs. FFTs.  The results have all been incorporated into the Great3 Handbook, and most of the code changes that were requires along the way have already been merge in elsewhere.  So I don't think there is much to review here. 

However, there are a couple changes to the HSM code, some utility functions, and a couple new unit tests.  So I figure it's worth going through the normal process with a PR in case anyone would like to comment on any of this before merging to master.