Suggestion: add pbd_is_monophyletic and pbd_is_paraphyletic

[richelbilderbeek]

I created a function in R to detect if a phylogeny has a paraphyly or not (see https://github.com/richelbilderbeek/R/blob/master/Phylogenies/is_monophyletic.R ). You think it is a good idea to add it to PBD?

[richelbilderbeek]

Or put the function used by the script check_nonmonophly.R (supplement of the submitted paper with a.o. James Rosindell)