Out of node access to variables in .ds

[TomNicholas]

Intended to close part (2) of #80, but currently raises a RecursionError.

Allow path-like access to DataArray objects stored in other nodes via getitem

• [x] Closes #80
• [x] Tests added
• [x] Passes pre-commit run --all-files
• [ ] New functions/methods are listed in api.rst
• [ ] Changes are summarized in docs/source/whats-new.rst

[TomNicholas]

Based on discussion in https://github.com/xarray-contrib/datatree/pull/99#discussion_r887004389 we will leave this for now, and see if it's actually something that users particularly want later.