Error on PhyloXML documents with multiple phylogenies

[miguel76]

As of version 1.20 (https://www.phyloxml.org/documentation/version_1.20/phyloxml.html) PhyloXML supports multiple phylogenies in the same file.
When a PhyloXML with multiple phylogenies is parsed at the following line tree_json becomes undefined because xml.phyloxml.phylogeny is an array rather than an object:

phylotree.js/src/formats/phyloxml.js

Line 70 in 60aa174

tree_json = xml.phyloxml.phylogeny.clade;

That leads to an error at the following line:

phylotree.js/src/formats/phyloxml.js

Line 71 in 60aa174

tree_json.name = "root";

This case should be managed in a clear way, either throwing an error stating that multiple phylogenies are not supported or choosing one of the phylogenies (e.g., the first) and giving a warning.

Ideally, it would be nice to load multi-phylogenies files as a set of Phylotree objects, but that would be an improvement rather than just fixing the bug, which is the priority in my opinion.