rarefaction dens_ratio argument

[dmcglinn]

it would make it easier for users to compute the non-spatial sampled based rarefaction curve if they only have to input the reference density and not the argument dens_ratio which is the ratio of the reference density to observed density. The observed density can be simply calculated from the input community data.

[sablowes]

+1. I have always found the dens_ratio hard to wrap my head around!