Add 'stable' flag to group_by operation (#1102)

* add 'stable' flag to group_by

* add stable option to DataFrame.frequencies

* make grouped operations non-stable by default

* move groups related data to separate map in DataFrame struct

* remove outdated TODO

Author: petrkozorezov

lib/explorer/backend/data_frame.ex

@@ -275,7 +275,7 @@ defmodule Explorer.Backend.DataFrame do
   end
 
   def new(data, names, dtypes) when is_list(names) and is_map(dtypes) do
-    %DataFrame{data: data, names: names, dtypes: dtypes, groups: []}
+    %DataFrame{data: data, names: names, dtypes: dtypes, groups: %{columns: [], stable?: false}}
   end
 
   @default_limit 5
@@ -300,7 +300,7 @@ defmodule Explorer.Backend.DataFrame do
       open,
       df_info,
       close,
-      groups_algebra(df.groups, inspect_opts) | cols_algebra
+      groups_algebra(df.groups.columns, inspect_opts) | cols_algebra
     ])
   end
 

lib/explorer/backend/query_frame.ex

@@ -74,7 +74,7 @@ defmodule Explorer.Backend.QueryFrame do
       open,
       "??? x #{length(cols_algebra)}",
       close,
-      groups_algebra(ldf.groups, opts) | cols_algebra
+      groups_algebra(ldf.groups.columns, opts) | cols_algebra
     ])
   end