Allow for multiple stratification levels at once during functional classification

[droush]

Taxonomic classification can be multiplexed to do multiple ranks at once, including the generation of read-to-rank maps for rank given rank.

For example,

woltka classify -i samdir -o . \
  -m taxid.map --nodes nodes.dmp --names names.dmp \
  -r none,free,phylum,class,order,family,genus,species \
  -u map_dir

However, a user is unable to pass a directory containing multiple read-to-rank maps to the classify function for functional stratification at multiple ranks.

i.e.

woltka classify \
  --input sam_dir \
  --coords coords.txt.xz \
  --map wol-to-protein.txt.xz \
  --names metacyc/23.0/protein_name.txt \
  --stratify read-to-rank-maps \
  --map-as-rank \
  --rank protein \
  --output protein.biom

I imagine we could implement this relatively easily, with some modifications for specifying output to take into account multiple ranks.