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.
Maybe just have a nice docstring for `_delegate_to_h5file`? I feel like there's a lot of copypasta (:spaghetti:) and opportunity for rot by having mostly the same docstrings in this class.