Try removing the N parameter from Signal

Keeping track of the number of channels makes type stability very tough.  It make some methods easier, but the only thing I've found it useful for dispatch is display.

Note that in other domains, e.g., Audio, this may be more important.  We can always add it back in if it will be useful.
Fix irregular windows of multiple channels

Use matrices of Signals
Restructure Signal types to be more explicit

Use an abstract RegularSignal to make dispatch simpler.  For now, only support Vector channels, but I think this should allow a MatrixSignal type without much hand-wringing.