(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/autocomplete_context.py

@@ -35,7 +35,7 @@ class AutocompleteContext(object):
     # TODO It would be nice to include builtin functions too, but getargspec doesn't work there.
     self._functions = {}
     for key, value in six.iteritems(self._context):
-      if value and callable(value):
+      if value and callable(value) and not isinstance(value, type):
         argspec = inspect.formatargspec(*inspect.getargspec(value))
         self._functions[key] = Completion(key, argspec, is_grist_func(value))