Flatten a multiphylo object
Details
NeXML has the concept of multiple <trees> nodes, each with multiple child <tree> nodes.
This maps naturally to a list of multiphylo objects. Sometimes
this hierarchy conveys important structural information, so it is not discarded by default.
Occasionally it is useful to flatten the structure though, hence this function. Note that this
discards the original structure, and the nexml file must be parsed again to recover it.