feat(g2p): add gget g2p module for the Genomics 2 Proteins portal (#138)

[Elarwei001]

Summary

Resolves #138. Adds a new module gget g2p that queries the Genomics 2 Proteins (G2P) portal (Broad Institute; Kwon, Safer, Nguyen et al., Nature Methods 2024) to link genes/proteins to residue-level structural and functional annotations.

The G2P REST API serves tab-separated values, which the module parses into a pandas DataFrame — fitting gget's "query a database → DataFrame in one line" idiom (cf. gget pdb, gget bgee).

What it does

gget.g2p(gene, uniprot_id, resource="features"|"map"|"alignment", isoform=None, save=False, verbose=True)

• features (default): per-residue feature table (AlphaFold pLDDT, UniProt sites, secondary structure, predicted pockets, PTMs, …) — 140+ columns.
• map: gene → transcript → protein isoform → structure map (UniProt / Ensembl / RefSeq / PDB identifiers).
• alignment: residue-level sequence alignment between two isoforms (requires isoform; uniprot_id is the canonical isoform).

gget g2p BRCA1 -u P38398                 # per-residue features (JSON)
gget g2p BRCA1 -u P38398 -r map --csv    # isoform/structure map (CSV)
gget g2p LDLR -u P01130-1 -r alignment -i P01130-2

Changes

• gget/gget_g2p.py — new module (direct REST via requests, TSV → DataFrame; no new heavy dependency, does not vendor the g2papi client).
• gget/main.py — parser_g2p + dispatch (positional gene, -u/--uniprot_id, -r/--resource, -i/--isoform, -o/--out, -csv, -q).
• gget/__init__.py — export g2p; gget/constants.py — G2P_API.
• tests/test_g2p.py — live integration tests (assert on stable columns / identifiers, since the feature table is wide and its values can change) + network-free argument-validation tests.
• docs/src/en/g2p.md + docs/src/en/updates.md.

Testing

All tests pass locally (Python 3.11), and the CLI was exercised for all three resources:

$ pytest tests/test_g2p.py -v
...
6 passed

Notes

• Endpoints/fields were confirmed against the live G2P API (the g2papi README suggested JSON, but the API actually returns TSV — verified and handled accordingly).
• uniprot_id is required; the help text and error message point users to gget info to find it. Auto-resolving UniProt IDs from a gene symbol could be a follow-up.
• Rate limits are undocumented; the module makes a single request per call.