Fermi source data

Can these be added? I want to use them for the unit tests & tutorial for npred cube and needed a single clear source to do a good comparison between my npred method (see PR #151) and the fermi science tools (so couldn't just use the FermiGalacticCenter stuff). Introduces:
357.1 kB counts_vela.fits.gz
1.5 kB exposure_vela.fits.gz
Are these an OK size to include? I could probably reduce the resolution + size of the counts image once I've written the tutorial & know what is needed there.

I picked the vela region as I already had these files to hand. Could use a different region if preferred.
Rename `background_cube` -> `background_image` (I was surprised to see in the test that it's 2d.