Add open_mfdatatree

What is your issue?

Currently we have an open_datatree function which opens a single netcdf file (or zarr store). We could imagine an open_mfdatatree function which is analogous to open_mfdataset, which can open multiple files at once.

As DataTree has a structure essentially the same as that of a filesystem, I'm imagining a use case where the user has a bunch of data files stored in nested directories, e.g.

project
    /experimental
        data.nc
    /simulation
        /highres
            output.nc
        /lowres
            output.nc

We could look through all of these folders recursively, open any files found of the correct format, and store them in a single tree.

We could even allow for multiple data files in each folder if we called open_mfdataset on all the files found in each folder.

EDIT: We could also save a tree out to multiple folders like this using a save_mfdatatree method.

This might be particularly useful for users who want the benefit of a tree-like structure but are using a file format that doesn't support groups.

Originally posted by @TomNicholas in https://github.com/xarray-contrib/datatree/issues/51#issue-1082703410

Start by locating the existing open_datatree entry point and comparing it with open_mfdataset. Define how recursive directories, supported file formats, and multiple files per folder should map into one DataTree, then verify the chosen behavior with tests for the nested example and the issue's proposed save_mfdatatree counterpart.