mob_stats breaks when group_var is not a factor

[dmcglinn]

This is because of this line which assumes that group is a factor. We need a more robust system because group could be a character or a numeric class of variable.

[FelixMay]

I started debugging the mob_stats function. During that I noticed that in the example data set inv_comm there are sites with one or even zero individuals. In this case of course rarefaction does not show any interesting results.
Any ideas how to deal with this issue? Only in this function by selecting a certain minimum number of individuals or is this relevant also for other functions?

[FelixMay]

And a second questions on revision of this function (mob_stats) what about the extrapolated richness using SpadeR::ChaoSpecies? Should I reintroduce that or does that cause problems because of package dependencies? What about using vegan::estimateR ?