needs completion and testing...
rarefy update. Needs (added?) unit testing still.
v1.5.16 Upgrades to rarefy_even_depth() function

More user feedback about process.
New argument for setting RNG seed, with a default value (reproducible
by default).
Reports the RNG seed being used for random subsampling, encouraging
users to record this for reproducibility.
Trims empty samples/OTUs automatically, and reports this to standard
out.
Merge pull request #214 from joey711/rarefy-features

v1.5.16 improvements for `rarefy_even_depth()` function