POC immunedeconv as functional method and fix error for custom column

[delfiterradas]

This is a POC to make immunedeconv a functional method (#469) and also add automatic detection and conversion of ENSEMBL gene IDs (human/mouse) to gene symbols, with safe fallback to the first column if gene_symbol_col is missing (#512).

PR checklist

• This comment contains a description of changes (with reason).
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• Output Documentation in docs/output.md is updated.
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