(core) CONTAINS() and summarising by ChoiceList columns with flattening

Summary:
Added CONTAINS 'function' which can be used in lookups

Changed LookupMapColumn._row_key_map to use right=set so one row can have many keys when CONTAINS is used.

Use CONTAINS to implement group column in summary table, while helper column in source table can reference and create multiple rows in summary table, especially when summarising by ChoiceList columns.

Use itertools.product to generate all combinations of lookup keys and groupby values.

cleanup

Test Plan: Added python unit tests.

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: paulfitz, dsagal

Differential Revision: https://phab.getgrist.com/D2900

sandbox/grist/functions/lookup.py

@@ -1,5 +1,5 @@
 # pylint: disable=redefined-builtin, line-too-long
-from collections import OrderedDict
+from collections import OrderedDict, namedtuple
 import os
 
 import six
@@ -150,3 +150,25 @@ def VLOOKUP(table, **field_value_pairs):
   ```
   """
   return table.lookupOne(**field_value_pairs)
+
+class CONTAINS(namedtuple("CONTAINS", "value")):
+  """
+  Use this marker with `Table.lookupRecords` to find records
+  where a column contains the given value, e.g:
+
+      MoviesTable.lookupRecords(genre=CONTAINS("Drama"))
+
+  will return records in `MoviesTable` where the column `genre`
+  is a list or other container such as `["Comedy", "Drama"]`,
+  i.e. `"Drama" in $genre`.
+
+  Note that the column being looked up (e.g. `genre`)
+  must have values of a container type such as list, tuple, or set.
+  In particular the values mustn't be strings, e.g. `"Comedy-Drama"` won't match
+  even though `"Drama" in "Comedy-Drama"` is `True` in Python.
+  It also won't match substrings within container elements, e.g. `["Comedy-Drama"]`.
+  """
+  # While users should apply this marker to values in queries, internally
+  # the marker is moved to the column ID so that the LookupMapColumn knows how to
+  # update its index correctly for that column.
+  pass