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.
It's not true that you are returning a `~gammapy.spectral_cube.GammaSpectralCube` ... at the moment it's an `HDU` object. I'm not sure which is better, ... you decide what is nicer for your use case, but make the docstring and code consistent.