Possible small bug in makemask in reduce/calib.py

[cdfassnacht]

In looking through the code for the makemask function in calib.py, I noticed what may be a small cut-and-paste bug. In line 297 (as seen by looking at the file on github.com using a web browser) the flat_stats variable is defined by a call to stats.sigma_clipped_stats. I think that this should give statistics for the flats, but the input list is img_dk rather than img_fl, as I would expect and as is used in the else portion of this if/else structure. Perhaps this bug never causes problems if people are using up-to-date versions of astropy, but I thought I should bring it to your attention nonetheless.