pytree: support nested vectors

[cshimmin]

Currently pytree can infer nested types and create branches for them, but I need to do a bit more work to actually automate the write-out. Mostly this is problematic because nested vectors implies you'll have to create new vector objects for every fill. And often this is not exactly what we want; it would sometimes be better to only clear()/push_back() on the innermost vectors. How to deal?