Use vegan in rarefy

[TuomasBorman]

Should we switch using vegan::rrafefy (it is fast since it is programmed with C)? One downside is that it does not support replace. Moreover, one thing to discuss (as I am not the expert of the terminology), could it make more sense to do iterations? As I have understood, the criticism towards rafefying is mostly related to bias and missing data caused by rarefaction single time. However, if enough iterations is done, that would solve the issue.

@antagomir