Spectral cubes and line maps from star-forming regions in M31.  Given to
us by Kathryn Kreckel in Feb 2020.

Data came in by rsync.

Upstream had large areas of NaNs in their data.  Since they were a
big waste of space, I've taken them away; however, autodetected crop
borders were a bit different for the different images, so bin/cropfits.py
is a two-stage operation.  See comment at the top.

The bin/addheader.py processor can run before or after that.

Test set: PPAK_M31_F4_OIII5007.fits and PPAK_M31_F4_cube.fits, so far
uncropped.