[codex] Check MHCflurry percentile calibration

[iskandr]

Summary

• keep MHCflurry affinity percentile ranks enabled by default
• preflight requested alleles for affinity percentile-rank calibration before prediction
• accept calibration from any allele with the same MHCflurry pseudosequence
• add include_affinity_percentile_ranks=False for callers that want raw affinity without affinity ranks
• bump mhctools to 3.13.4

Fixes #203.

Validation

• python -m pytest tests/test_mhcflurry_key_lookup.py tests/test_mhcflurry.py
• ./lint.sh
• ./test.sh
• manual repro: MHCflurry(alleles=["HLA-C*15:05"]) now fails early with a clear calibration error
• manual opt-out: MHCflurry(alleles=["HLA-C*15:05"], include_affinity_percentile_ranks=False).predict(["SLYNTVATL"]) returns raw affinity with no affinity percentile rank