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.
I couldn't find a way to keep these lines below the 80 character limit and still build OK in the docs. Do you know how I can do this?