Add support for space group information when reading and writing pdb files

[alex-sbaq]

Added get_space_group() and set_space_group() methods as discussed in #689 to support space group and Z value reading/writing for (deprecated) PDB files.

[padix-key]

Hi, thanks for implementing this functionality! Could you add a test as well to tests/structure/io/test_pdb.py? I think something analogous to test_conversion() would work well here: Setting and subsequent retrieving of the space group should return the original space group.

[codspeed-hq]

CodSpeed Performance Report

Merging #707 will not alter performance

_{Comparing alex-sbaq:pdb_space_group (37cfb46) with main (2765f4b)}

Summary

✅ 47 untouched benchmarks

[padix-key]

Thanks for the update! I will give the PR a final review on the weekend.

[padix-key]

Thanks for adding the test. I have some minor requests for it, but otherwise, everything looks good!

[alex-sbaq]

Thanks for the feedback, @padix-key! I've made the changes to the test - let me know if anything is missing.