add functions for rarefaction for groups of signatures using vegan

[taylorreiter]

like used here (note all of these links are to the specaccum branch which will be deleted after https://github.com/Arcadia-Science/2022-mtx-not-in-mgx-pairs/pull/9 is merged):

• https://github.com/Arcadia-Science/2022-mtx-not-in-mgx-pairs/blob/ter/specaccum/scripts/sig_to_csv_abund.py
• https://github.com/Arcadia-Science/2022-mtx-not-in-mgx-pairs/blob/ter/specaccum/scripts/calc_rarecurves.R
• https://github.com/Arcadia-Science/2022-mtx-not-in-mgx-pairs/blob/ter/specaccum/notebooks/20220929-visualize-most-converged-rarefaction-curves.ipynb

Only makes sense to run signatures with abundances calculated from reads. Also only really makes sense when it's run on many signatures from the same sample.

requires signatures to be read into a data frame (see #4)

[taylorreiter]

Signatures read in in #16. Rarefaction curves implemented in #17.

[taylorreiter]

closed by #17