Add King distribution as spatial PDF

[chiarabellenghi]

Michael implemented it for csky. There is a draft PR at the moment: https://github.com/icecube/csky/pull/207.

It could be a nice and efficient replacement for the ultra-complex KDE implementation if it also produces unbiased MLEs.

For starters, I would like to test this on the NT v005p04.

[mjlarson]

FYI, there's a cleaner version in a separate project at https://github.com/mjlarson/kingmaker. I haven't finalized it yet since I still have to incorporate the fitting code, but that project is intended to be a more generic replacement usable across more fitting codes. It already incorporates the base PDF as well as signal subtraction and template smearing.

If you're interested, maybe we can find some time to chat sometime once my workload calms down. I'd like to get it finished off, add documentation and fitting code, and incorporate it into csky, skyllh, and the realtime software.

And yes, it's VERY barebones! I know! Sorry! Been too busy to finish it for the last ~6-10 months...

[chiarabellenghi]

Cool! I'll certainly take a look. Not sure when, given the length of the list of issues that we are creating...
But yes. Let's chat about it when we find a minute!