This is a non-traditional brainstorming RFC to gather preliminary input prior to design work.  The (mostly-)completed design will be RFC'd separately in the future (and I'm not even planning to start that work for several weeks).

My goal is to replace afw::image::Filter with something that:

• minimizes the use of singletons;
• maps to the Gen3 PhysicalFilter and AbstractFilter concepts (probably with separate classes for these);
• has a sensible relationship with cameraGeom (probably just "Camera has a set of PhysicalFilters");
• has a sensible relationship with TransmissionCurve (not obvious; could be "PhysicalFilter has TransmissionCurve" or "PhysicalFilter can be used to retrieve a TransmissionCurve from a calibration repo");
• can be easily mangled into deterministic integer IDs without addition or subtraction of filters breaking old IDs;
• natively supports or can be extended to support sub-filters for DCR-correctable coadds.

 More use cases and design ideas welcome.