Assumed file name may result in an error when generating naive maps

[petiay]

(This is minor). Although the standard BEAST outputs are stored in a stats.fits file, referring to stats.fits in this line of make_naive_maps.py may result in an error if one has changed the name of the output file where stats.fits does not exist in a file name. (Actually the error I encountered ends up overwriting the very stats.fits file! )
For this reason I'd lobby for only referring to .fits instead of to stats.fits, unless values_per_pixel.hd5 without a preceding stats is an expected input somewhere else at this point and it's hard to change that.

[christinawlindberg]

I've run into this same issue, where, because my stats files had a suffix, the code just ends up overwriting my original file.