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.
Make this shorter to fit on one line ... e.g.:
```
* ``P7REP_SOURCE_V15_68``: P7REP_SOURCE_V15, 68% containment