Add tips according to csv or tsv file of taxon names and taxa
that form the clade into which you"d like to insert the tip. One column
should be called 'taxon', and should contain the taxon to be placed. The
other column should be called 'clade' and contain the taxon with which the
taxon to be placed will form a group. Each member of the clade will be
placed on its own line. An example of this file can be seen in `inst/extdat
a/mrca_df.tsv`