Add APP PSF

[hugobuddel]

From @astronomyk

Here's my first attempt at the APP PSF for the L-band.
To us it, all you need to do is download it into the same folder as the other METIS PSF (i.e. in the inst_pkgs/METIS folder), and then update the METIS config parameter "!OBS.psf_file" with the name of the new file. Something like this:

metis.cmds["!OBS.psf_file"] = "METIS_PSF_L_APP.fits"

should work. I'll try to test it myself a little bit later today.

@astronomyk should we keep this PSF around in the IRDB? That is the most logical place to keep it.