Bug/documentation issue: Requesting altloc ("label_alt_id") in `extra_fields` gets overwritten by `"first"` if occupancy is not also in `extra_fields`

[k-chrispens]

https://github.com/RosettaCommons/atomworks/blob/c314ddd1a069f3e7d051a59f24fbfc908f51c514/src/atomworks/io/utils/io_utils.py#L279C17-L279C83

I ran into an issue using load_any to parse a mmCIF file into an AtomArrayStack.

>>> # This is the mmCIF with PDB ID 6B8X downloaded from RCSB
>>> st = load_any('tests/resources/6b8x/6B8X.cif', altloc='all', extra_fields=["label_alt_id", ]) 
>>> # We know this has non-hetatm alt B records
>>> st[0].query("~hetero and label_alt_id == 'B'")
AtomArray([
])

The issue appears to be in the atomworks version of get_structure, which silently assumes the altloc is "first" unless occupancy is also specified as an extra field. This should be documented or fixed.

Preferably, this would return with "first" only if no additional altloc/occupancy related extra fields are provided.

[marcuscollins]

To add details, the issue is at https://github.com/RosettaCommons/atomworks/blob/production/src/atomworks/io/utils/io_utils.py#L279
Probably this should either 1) raise an error if the required extra fields ("occupancy") aren't present when using altloc "all" or "occupancy", or 2) add the extra field "occupancy" to the extra fields for altloc = "all" | "occupancy"