Mouse_Hexamer.tsv and Human_Hexamer.tsv are the published CPAT hexamer
frequency tables (Wang et al. 2013). TxNova uses the coding and noncoding
frequencies for an in-process hexamer log-likelihood. It does not run CPAT
and does not use CPAT's logistic model. `species: auto` infers mouse or
human from the GTF; `mouse` / `human` select the table. Only those two
are supported. `coding.hexamer_table` overrides the packaged file.
