Infer hetero residue IDs from author annotation

[padix-key]

As discussed in #773, This PR creates continuously incrementing res_ids for hetero residues in a structure loaded with pdbx.get_structure(use_author_fields=False).
Previously, those hetero residue IDs would be [-1, -1, -1, ...] making it impossible to infer residue boundaries with structure.get_residue_starts() (#553). With this PR those residue IDs would be [1, 2, 3, ...].

Fixes #553.

[codspeed-hq]

CodSpeed Performance Report

Merging #823 will not alter performance

_{Comparing padix-key:issue-553 (e4afe9b) with main (7474d90)}

Summary

✅ 59 untouched benchmarks