Taxon traits are contained in a data frame at the slot 'taxonTraits'
in
'>taxlist
objects.
To optimise space, this data frame contain only entries for those concepts with
information, while taxa with no information are skipped from this table.
Thus appending new variables may also have to include new rows in this slot,
which is automatically carried out by this function.
The replacement method 'taxon_traits<-'
should be only used when
constructing '>taxlist
objects from an empty one.