GalSim: add formula for GalSim 0.5: galaxy image simulation toolkit

GalSim is a package for simulating astronomical images of galaxies.
It was developed to support the upcoming "Great3" challenge,
http://great3challenge.info/, to test different methods for measuring
weak gravitational lensing, a key way of measuring mass in the universe
through its bending of light.



With the Great3 challenge set to begin this month, it would be great to have GalSim in the main homebrew-science tap.  I have been maintaining it in our fork, https://github.com/camphogg/homebrew-science/blob/master/gal-sim.rb for several monhts.  Probably 98% of professional astronomers use Mac laptops, so it would be great to have gal-sim in the mainline homebrew-science tap.

I am not a GalSim developer, though I do share an office with one.