Better editing support

[bovee]

It should be possible to:

• add a new node.
• edit the name and rank on an existing node
• update the parent on a node
• delete a node (whether this should delete all the child or just move them to the parent is an open question; probably the latter because we do already have a prune method)

Editing could happen either via dedicated methods like tax.add(parent_id, new_id, name=None, rank=None) or via some kind of tax[new_id] = (parent_id, name, rank) access (not this should probably be polymorphic so you can just do tax[new_id] = parent_id too although this won't help with setting only the e.g. name).