Generic Python wrappers that turn Algorithms into Plugins
Use generic wrappers for PsfFlux plugins
Utility classes for safely getting centroids and shapes from slots
Add trivial enum to FlagHandler to establish convention for general failure flag
Use FlagHandler trivial enum in PsfFlux.
Fixup generic algorithm wrappers by using staticmethod descriptor on factory
Use pex_exceptions macro to define FatalAlgorithmError

Unlike MeasurementError, this doesn't have any extra data members or methods,
so we should have been using this macro from the beginning to cut down on
boilerplate.
Move PsfFlux measure methods to private to avoid shadowing
Use centroid extractor in PsfFlux