add gal-sim: a galaxy simulation toolbox.

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.

The Great3 challenge is in progress, so 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.

See also https://github.com/Homebrew/homebrew-science/pull/544
which adds prerequisite tmv-cpp