Reimplement DataTree aggregations (#9589)

* Reimplement DataTree aggregations

They now allow for dimensions that are missing on particular nodes, and
use Xarray's standard generate_aggregations machinery, like aggregations
for DataArray and Dataset.

Fixes #8949, #8963

* add API docs on DataTree aggregations

* remove incorrectly added sel methods

* fix docstring reprs

* mypy fix

* fix self import

* remove unimplemented agg methods

* replace dim_arg_to_dims_set with parse_dims

* add parse_dims_as_set

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix mypy errors

* change tests to match slightly different error now thrown

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: TomNicholas <tom@cworthy.org>

doc/api.rst

@@ -828,30 +828,26 @@ Index into all nodes in the subtree simultaneously.
 ..    DataTree.polyfit
 ..    DataTree.curvefit
 
-.. Aggregation
-.. -----------
+Aggregation
+-----------
 
-.. Aggregate data in all nodes in the subtree simultaneously.
+Aggregate data in all nodes in the subtree simultaneously.
 
-.. .. autosummary::
-..    :toctree: generated/
+.. autosummary::
+   :toctree: generated/
 
-..    DataTree.all
-..    DataTree.any
-..    DataTree.argmax
-..    DataTree.argmin
-..    DataTree.idxmax
-..    DataTree.idxmin
-..    DataTree.max
-..    DataTree.min
-..    DataTree.mean
-..    DataTree.median
-..    DataTree.prod
-..    DataTree.sum
-..    DataTree.std
-..    DataTree.var
-..    DataTree.cumsum
-..    DataTree.cumprod
+   DataTree.all
+   DataTree.any
+   DataTree.max
+   DataTree.min
+   DataTree.mean
+   DataTree.median
+   DataTree.prod
+   DataTree.sum
+   DataTree.std
+   DataTree.var
+   DataTree.cumsum
+   DataTree.cumprod
 
 .. ndarray methods
 .. ---------------