Refactor group and node manipulation from H5File down into H5Group

This PR allows H5Groups to create and remove subgroups directly without requiring the user to manually dig out `_h5_file`.

It also extends the behavior of most functions creating new groups and nodes to also return those nodes.
Hmm, I guess you're doing this because `_delegate_to_h5file` only has the `node_subpath` argument. Ignore these two suggestions.