DataTree: fetch inherited coords on demand

[TomNicholas]

This takes the new tests from #9063 and makes (most of) them pass using a different implementation. I didn't get to the IO part but this should be enough to show you what I was imagining.

cc @shoyer

@keewis @owenlittlejohns @flamingbear @eni-awowale I'm interested if any of you see advantages in either mine or @shoyer's approach here (or don't care)

• [x] Alternative implementation of #9063 as a solution to #9056
• [x] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

[TomNicholas]

Closing as replaced by #9063