Add better handling of name conflicts in internal methods

[mikemc]

For example, currently the taxonomyTable method for merge_taxa_vec() assumes that there are no taxonomic ranks named "group" or "taxon"

[mikemc]

As a quick fix, could perhaps use ...group and ...taxon to make conflicts very unlikely, and exit with an error if these would cause a conflict.

[mikemc]

Instead of ...taxon, might instead use .otu since this is the special column name being used by new ps_tibble() and dplyr verb functions