Add character data to a nexml object

Usage

add_characters(x, nexml = new("nexml"), append_to_existing_otus = FALSE)

x: character data, in which character traits labels are column names and taxon labels are row names. x can be in matrix or data.frame format.
nexml: a nexml object, if appending character table to an existing nexml object. If omitted will initiate a new nexml object.
append_to_existing_otus: logical. If TRUE, will add any new taxa (taxa not matching any existing otus block) to the existing (first) otus block. Otherwise (default), a new otus block is created, even though it may contain duplicate taxa to those already present. While FALSE is the safe option, TRUE may be appropriate when building nexml files from scratch with both characters and trees.