Recover Sherpa analysis status when fitting PCA backgrounds

[ruizca]

This pull request address issue #34

[JohannesBuchner]

This looks good. Nice and clean code.

Did you test it?

I have to set up Github actions first, so that this can be tested (since the last BXA release, travis-CI removed their permanently-free open source tier).

But I wonder if the alternative approach you suggested, may not be better, because it requires less code to maintain. By that I mean documenting the behaviour so that users are aware. That would be editing this document: doc/pca-background-models.rst