WIP: check pairwise vs sourmash compare output

[ctb]

This PR evaluates and compares the results of sourmash compare to the output of pairwise.

Specifically, it:

• adds a directory, test_check_pairwise, containing a snakemake workflow;
• this workflow uses both sourmash compare and pairwise from this plugin to calculate comparisons between the 64 podar-ref sketches.
• the matrix output file is then converted into a "pairwise"-style CSV file, and a separate script compares the values to make sure they're the same/similar enough.

At least at the moment, the comparison is done only for the average_containment_ani column.

It is analogous to the gather check from https://github.com/ctb/2024-debug-gather-difference that we will add to this repo too, ref #363.

See sourmash-bio/sourmash#3134 for motivating inquiry ;)

TODO:

• consider cleaning up code
• consider joining forces with gather check
• consider adding to CI