`__getitem__` and `__setitem__` format

[bovee]

Right now we only expose name, rank = tax[tax_id] in Python, but we should add a way to set properties via indexing for ergonomics. I think this could either take the form of:

• a dedicated TaxonomyNode object that mediates editing with the taxonomy (internally it would probably keep a reference the same way the TaxonomyIterator does)
• a namedtuple as the return and treating it the same was as a dedicated object above
• a regular tuple as the return and some sort of coercion on setting from other tuples with fewer element; this seem more brittle than the other options