Changes the label for taxon `i` to `s` The index `i` should be 0-based.
|NxsNCLAPIException ||if `i` is out of range. |
|DuplicatedLabelNxsException ||if the label is already in the block |
|NxsException ||if the label is not a legal taxon name (eg. it is a punctation character). |
Implemented in NxsTaxaBlock.