Datatree: API for collapsing subtrees

What is your issue?

Originally posted by @TomNicholas in https://github.com/xarray-contrib/datatree/issues/192

Multiple people have requested methods for collapsing children or subtrees into single nodes.

Following on from our discourse discussion @abkfenris @patrickcgray

Also see https://github.com/pydata/xarray/issues/3996#issuecomment-1373993285 @rabernat @

How about an API like this?

class DataTree:
    def merge_subtree(self, **merge_options) -> DataTree:
        ...

    def concat_subtree(self, **concat_options) -> DataTree:
        ...

    def combine_subtree(self, combine="by_coords", **combine_options) -> DataTree:
        ...

or maybe even the _subtree suffix is superfluous...

Then the usage might be

dt['/path/to/node'] = dt['/path/to/node'].merge_subtree()

You could chain this with the new .filter() too which would be neat.

Start with the DataTree API proposed in the issue, then read the linked discourse discussion and xarray issue for context. Clarify whether merge_subtree, concat_subtree, combine_subtree, and the _subtree naming are required; done means the API scope and expected behavior are agreed before implementation begins.