Use of multiple groups for same species in `psdAdd()` and `wrAdd()` et al.

[droglenc]

Utility issues occur if the user needs to apply separate "groups" to the same "species" in the same data set. For example, this would occur if the data set contained both lotic and lentic Brook Trout. The use of group= is not flexible enough for these cases and thus limits the use of psdAdd() and wrAdd().