add method for deep member retrieval

[d-v-b]

Zarr python uses group[a/b/c] to get the node at <group>/a/b/c. In pydantic-zarr, you have to do group.members['a'].members['b'].members['c'], which is a lot more verbose (but also more type-safe, if the type of members is declared in advance). We could support these queries with a method on groups that takes /-delimited strings and does nested member access. Not sure yet what it could be called. get_subnode seems fine.