Add functions to produce upset plots and rarefaction plots from sourmash signatures

[taylorreiter]

This PR makes functions to visualize groups of sourmash signatures. Upset plots show intersections of shared minhashes between samples. Rarefaction curves show sequencing depth saturation for a sample.

• Motivating example of rarefaction curves:
  • code to make data in plot: https://github.com/Arcadia-Science/2022-mtx-not-in-mgx-pairs/blob/ter/specaccum/scripts/calc_rarecurves.R
  • Actual plot: https://github.com/Arcadia-Science/2022-mtx-not-in-mgx-pairs/blob/ter/specaccum/notebooks/20220929-visualize-most-converged-rarefaction-curves.ipynb
• Motivating example of upset plots: https://github.com/Arcadia-Science/2022-prjna853785-sourmash/blob/main/notebooks/20220812-visualize-sourmash-signature-intersect.ipynb

I found a bug in the read_signature code where I wasn't writing out the scaled value by default, so I also fixed that in this PR as I needed the scaled value for the plots.

I also figured out where to add descriptions to functions, so this PR has better documentation than previous ones. Eventually I'll go back and add descriptions to the other existing functions.