Replace usage of dictionaries in `format_gene_df`

`format_gene_df` uses dictionaries (named lists) to create new col names. But this is problematic when there are many:many orthologs. Switch over to using some sort of dataframe/filtering strategy instead.

Also, provide a warning to users when `gene_output` is "dict" or "dict_rev" AND `non121_strategy` is not "dbs".
