Implement TablePSF and Fermi PSF

@adonath The [Fermi PSF](https://gammapy.readthedocs.org/en/latest/api/gammapy.morphology.fermi_psf.FermiPSF.html) is given as a 1-dim radial density distribution `P(r)` where `r` is the offset to the center.

Is there a utility function to turn this into a 2-dim astropy kernel and / or model, so that it can be used for convolution / fitting?

If not, can you write it or do you have any pointers how I could implement this?

cc @ellisowen (needs convolution with the Fermi PSF to compute model images)