treering flats

[hsnee]

this should resolve #120

[coveralls]

Coverage decreased (-0.2%) to 70.379% when pulling 8a21672 on u/hsnee/treering_flats into e311a5f on master.

[jchiang87]

Thanks for this Husni. I'll do a more complete review in the next day or so, but I wanted to suggest now that it would be better to make a standalone make_flats.py script rather than adding this functionality to imsim.py. So I would request that you revert the changes to imsim.py and add a make_flats.py script to the bin.src folder.

[cwwalter]

This was completed in #132.