DataTree broadcasting

[TomNicholas]

What is your issue?

From https://github.com/xarray-contrib/datatree/issues/199

Currently you can perform arithmetic with datatrees, e.g. dt + dt. (In fact the current implementation lets you apply arbitrary operations on n trees that return 1 to n new trees, see map_over_subtree.)

However currently these trees must have the same structure of nodes (i.e. be "isomorphic").

It would be useful to generalise tree operations to handle trees of different structure. I'm going to call this "tree broadcasting" (not to be confused with array broadcasting).