More comments
Do full Summary & Percentiles on GroupBy

Add a SummaryPerRow class which uses the existing Summary2 logic, but
gets called from the Flow stuff.
Slight refactoring of Summary2 to allow easy use.
Move some implementation out of api/SummaryPage2 to Summary2.
Replace PerRow.clone() with PerRow.make().
trying some exec2 things
some exec2 stuff