add gal-sim: a galaxy simulation toolbox.
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.

Closes #545.

ge11232002/homebrew-science,aeantipov/homebrew-science,quantumsteve/homebrew-science,pmelsted/homebrew-science,dpo/homebrew-science,pditommaso/homebrew-science,heuermh/homebrew-science,maplesond/homebrew-science,philippbayer/homebrew-science,parcollet/homebrew-science,joshbode/homebrew-science,andrewjpage/homebrew-science,kozo2/homebrew-science,goxberry/homebrew-science,DabeDotCom/homebrew-science,ahundt/homebrew-science