Add checks to subset_*() commands

[mikemc]

Currently, no warning is given for typos such as subset_taxa(ps, phylum = "Bacteroidetes"). Instead, this command will simply run and not filter out anything. Contrast with dplyr::filter(), which gives a helpful error message suggesting to use == instead of =.

Adding such behavior seems unambiguously beneficial, but there is perhaps some value in the simplicity of keeping the subset_ commands as simple wrappers around base::subset().