Add a nemoMask script

[mattyowl]

Add a script under bin/ that takes as arguments:

• path to a map (for getting needed pixelization)
• path to a nemo .fits catalog
• output file name

and outputs a mask in .fits format (0 = masked, 1 = valid area; this is the format understood by nemo's pointSourceMask config file option). Options could be added for variable masking radius (e.g., based on flux). Options used could perhaps be recorded in the .fits header of the mask (if necessary - maybe at least the path to the source catalog that was used).

It may also be useful for the script to be able to load DS9 .reg files (if we want to mask arbitrary-shaped regions).