Allow writing out custom annotation fields to cif files

[Croydon-Brixton]

feat: allow writing out custom annotation to cif file

[codspeed-hq]

CodSpeed Performance Report

Merging #669 will not alter performance

_{Comparing Croydon-Brixton:feat/cif_write_custom_annotations (2fbadb0) with main (231eefe)}

Summary

✅ 44 untouched benchmarks

[padix-key]

Thanks for implementing the feature the feature! I would propose a slight change: Instead of a boolean flag to automatically include all custom annotations, I would propose an extra_fields parameter, where the included annotations are named, because

• it is more consistent with the extra_fields parameter in get_structure() and
• Explicit is better than implicit according to the Zen of Python.

[Croydon-Brixton]

Great points, thank you for the feedback @padix-key -- I'll address those and update the PR

[Croydon-Brixton]

✅ Alright @padix-key - I've changed the API to use the extra_fields parameter as you suggested and added a mini test for the reading & writing of custom annotation.

[padix-key]

This looks good, I would only propose a small change, to make the test more clear.

[padix-key]

I see, that you are not in CONTRIB.rst, yet. You are welcome to add yourself if you like.

[Croydon-Brixton]

Thank you -- it'd be an honour!
I've added myself & implemented your suggested change & rebased. Should be good to merge from my side, after CI passes -- you're welcome to squash the commits ofc