write_star --datadir and RELION3.1 optics propagation

[michal-g]

I am preemptively opening this pull request as adding this feature has been on my radar for a while so I am eager to get this into the codebase promptly!

Also happy to use this space to discuss and help out with the coding and the merge; for what it's worth I am also preparing to do a refactor of the ImageSource/StarFileSource/_MRCDataFrameSource stuff very soon, so if you are going to do more work on this I would focus more on functionality than getting the code cleaned up — which I can take care of down the line!