Fix PC conventions and add a changelog

[hakonanes]

• Fix the PC conventions by indexing correctly into the Fortran NumPy array t.
• Add a changelog, which can be used to communicate new features and the like to users, following Keep a Changelog. In the future, this changelog can be added as-is to a documentation page similar to kikuchipy's changelog.

I checked the supported vendor conventions in pyebsdindex.ebsd_index.EBSDIndexer.index_pats(), and they now work with the conventions used in kikuchipy, with one exception: The EMsoft convention here corresponds to the EMsoft v4 convention in kikuchipy... I believe it is correct here, and wrong in kikuchipy, because in EMsoft >= 5.0, the detector screen is viewed towards the sample, and xpc increases towards the right from the middle of the detector. Could you confirm that it's the EMsoft v5 convention you implemented?

Should close #3.

[drowenhorst-nrl]

Could you confirm that it's the EMsoft v5 convention you implemented?
In PyEBSDIndex I intended using the EMSoft v5 convention. +x moves right across the detector, +y moves up. Origin is at the center of the detector as noted here.
Thanks for the testing and correction. That col,row habit is going to take a long time to break.

[hakonanes]

In PyEBSDIndex I intended using the EMSoft v5 convention. +x moves right across the detector, +y moves up. Origin is at the center of the detector as noted here.

Great, then I'm certain we have it wrong in kikuchipy (but correct in the docstring text), both in the docstring equation EMsoft -> Bruker and the corresponding code, which previously was correct... @IMBalENce and I discussed this previously before making the change, but apparently had it wrong. Oh well.

Thanks for the testing and correction.

No problem, it's a joy using PyEBSDIndex, and equally so to contribute.