Add support for per-phylogeny nomenclatural code

[gaurav]

In the Phyx.js manuscript, we define a defaultNomenclaturalCodeIRI field (in table 1), which sets a default nomenclatural code for the entire Phyx document. However, we don't have a similar field for the phylogeny object (see table 4), which would allow us to set a nomenclatural code for each phylogeny. Should we add such a field to the phylogeny object? This should take less than a week to implement in phyx.js and Klados.