Add pyregion as a dependency for masking
masking module for creating flagmaps with weightwatcher

The make_flagmap() function allows the user to create a flagmap by
combining existing weightmaps and polygon flag regions (in DS9 format)
with weightwatcher. Currently I assume that the region files are
polygons to comply to with WW.
make_flagmap does not need original image path
Add flag_pixels function to masking

Sets flagged pixels in ``flag_path`` to NaN in FITS at ``image_path``