BUG/TST: transform and filter on non-unique index, closes #4620

This is a relatively minor change. Previously, filter on
SeriesGroupBy and DataFrameGroupby and transform on only
SeriesGroupBy referred to the index of each group's object.

This cannot work with repeated indexes, because some repeated
indexes occur between more than one group. Instead, use
{Series, DataFrame}GroupBy.indices array, an array of
locations, not labels.