General cleanups

[brettviren]

Some amount of "stale" code exists in WCT. It is not actively harmful but adds conceptual noise to the signal. This issue attempts to list things to be cleaned up when anyone might have some spare time (haha).

• The BinnedDiffusion.h and code that uses it in gen/ is an early, inferior convolution algorithm. It has never been used in production (to my knowledge) and instead the superior BinnedDiffusion_transform.h is the established algorithm.
• The entire sig/ subpackage. I believe @HaiwangYu had some initial intention to do something in there but that did not fully materialize (please correct me if that is wrong). Note, the "spng" branch holds a SP redo that is active.

Please add anything else to this list in comments.