Add an abstract base class for spatially-varying transmission curves

Add a C++ abstract base class (with Python wrappers) for classes that represent wavelength-dependent transmission (e.g. filter curves) as a function of position.

These should make it possible to use an implementation in one coordinate system (e.g. focal plane coordinates) to provide a transmission curve looked up via another set of coordinates (e.g. pixels) via a proxy subclass of the ABC.

Design probably deserves an RFC. Many conversations on this the appropriate design for this functionality have been started but none have converged; some prior art exists in Sims. Should defer discussions about functionality beyond what's needed for HSC filter curve coaddition as much as possible.

These objects will need to be persistable with afw::table::io.