datatree: Implement in-place binary ops

[TomNicholas]

I suspect this actually needs a different implementation, in order to handle error recovery properly.

I would suggest handling this like Dataset._inplace_binary_op, which builds result Dataset and then replaces the contents of the current dataset with those of the new dataset.

Originally posted by @shoyer in https://github.com/pydata/xarray/pull/9619#discussion_r1800471609