Style fixup.
Add alias for values module in test file.
DerivedCell for cells as functions of cells.
Extract logic for setting up one widget to a function.
Add runtime filter modification to MultistageChannelFilter.
GRC block definition for MultistageChannelFilter.

It would be better to have a general system for generating GRC XML from
ExportedState annotations, but that's nontrivial, so skipping it until
such time as we want to have GRC blocks for everything.
GRC file for MultistageChannelFilter exercising.

We ought to have an automated test but until I figure out how best to
define acceptance criteria for a digital filter this is what I've got.