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.
Yeah I wasn't sure about the best way to handle it. The docstrings aren't quite the same because of the difference in how the paths are handled, but they are of course *mostly* the same.