Datatree: API for collapsing subtrees

[flamingbear]

What is your issue?

Originally posted by @TomNicholas in xarray-contrib/datatree#192

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

Following on from our discourse discussion @abkfenris @patrickcgray

Also see #3996 (comment) @rabernat @

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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.