Psf data

Adds some Fermi-LAT PSF data.

This will be used in unit tests for the FermiGalacticCenter and FermiVelaRegion psf files. I will add these tests in another PR once this one is OK & merged.
The 95% containment radius code repeats the 68% containment radius code.
Put it in a function `plot_containment_radii()` and call it with a `fraction=68` or `fraction=95` argument.