Error on PhyloXML trees with single-branch clades

[miguel76]

If a PhyloXML contains at some level a clade with only a single branch below, an error is thrown at the following line:

phylotree.js/src/formats/phyloxml.js

Line 49 in 60aa174

node.clade.forEach(parsePhyloxml);

The problem is that the code expects the property clade to always be an array, while it is an object in the single branching case (due to how XML is converted to JSON).