adding stage for external osc calc

[philippeller]

This is a new PISA stage, that allows one to use an external oscillation probability calculation. We use this in some (non IceCube) work we do. But still would be nice to merge it into master to keep things in place

[LeanderFischer]

Finally had a closer look and modified it a bit. I removed todo related comments, better create an issue for that, please!

Also, there is some new conflicts, since this took so long :see_no_evil:. Feel free to merge, once these are resolved and the checks pass.

[LeanderFischer]

Thanks for catching this up with master. Again, I'm afraid out checks are broken right now, that's why I re-triggered them. Otherwise this is ready to merge, but we need to fix the checks first :grimacing:

[LeanderFischer]

Please pull from master once more, to fix the checks!