My understanding is that this is really the only difference between the two subsampling techniques. Do you have suggestions for how to improve the naming or documentation to make this clearer?

Note that this is being ported from what was in PyCogent, so I'm not sure if this was the intended behavior or not. @gregcaporaso did you write `subsample_multinomial`? It's hard to tell because this function was added to QIIME's `pycogent_backports` and then moved over to PyCogent, so the commit history isn't all that helpful...