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.
This is a bit weird ... `emin` and `emax` aren't really energies here.
Please use [this utiltity function](https://gammapy.readthedocs.org/en/latest/api/gammapy.spectrum.energy_bounds_equal_log_spacing.html#gammapy.spectrum.energy_bounds_equal_log_spacing) instead to create an equal-log-spacing grid of energies?

Instead of passing parameters to this function you can simply create the energies array here or make it a global variable.