Add aggregation as top-level import

Author: dcherian

docs/source/api.rst

@@ -39,6 +39,6 @@ Aggregation Objects
 .. autosummary::
     :toctree: generated/
 
-    aggregations.Aggregation
+    Aggregation
     aggregations.sum_
     aggregations.nansum

docs/source/user-stories/custom-aggregations.ipynb

@@ -665,7 +665,7 @@
     }
    ],
    "source": [
-    "from flox.aggregations import Aggregation\n",
+    "from flox import Aggregation\n",
     "\n",
     "agg_median = Aggregation(\n",
     "    name=\"median\",\n",

flox/__init__.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 # flake8: noqa
 """Top-level module for flox ."""
+from .aggregations import Aggregation  # noqa
 from .core import groupby_reduce, rechunk_for_blockwise, rechunk_for_cohorts  # noqa
 
 try: