Add test that shows that we need an unstructure_hook for DataTree

This is the simplest way to keep DataTree object the original.
Otherwise it would return a copy of some sort.

test/test_cattrs.py

@@ -58,3 +58,19 @@ def test_unstructure_xarray_tree_to_ascii():
     # The whole string will be in memory when doing this,
     # duplicating the data.
     assert f_dict == "1 2 3"
+
+
+def test_unstructure_xarray_tree():
+    x_arr = xr.DataArray([1, 2, 3])
+    x_set = xr.Dataset({"x": x_arr})
+    x_tree = xr.DataTree(x_set)
+    f = Baz(x=x_tree)
+
+    converter = Converter()
+    converter.register_unstructure_hook(xr.DataTree, lambda v: v)
+    f_dict = converter.unstructure(f)
+
+    # We expect that the default unstructure functionality keeps
+    # the xarray as is.
+    # This helps when finally converting the dictionary to MF6 input files.
+    assert x_tree is f_dict["x"]