Allow reindexing to sparse (#430)

* Support reindexing to a sparse array

* Fix benchmarks

* fix?

* fix benchmarks

Author: dcherian

asv_bench/benchmarks/combine.py

@@ -14,7 +14,11 @@ def _get_combine(combine):
     if combine == "grouped":
         return partial(flox.core._grouped_combine, engine="numpy")
     else:
-        return partial(flox.core._simple_combine, reindex=False)
+        try:
+            reindex = flox.ReindexStrategy(blockwise=False)
+        except AttributeError:
+            reindex = False
+        return partial(flox.core._simple_combine, reindex=reindex)
 
 
 class Combine:

flox/__init__.py

@@ -9,6 +9,8 @@
     groupby_scan,
     rechunk_for_blockwise,
     rechunk_for_cohorts,
+    ReindexStrategy,
+    ReindexArrayType,
 )  # noqa