Registry and RegistryField for configuring subtasks

Example of a RegistryField created from a Registry object and use of both the Registry.register method and the registerConfigurable decorator:

psfDeterminerRegistry = pexConfig.makeRegistry("""A registry of PSF determiner factories""")

class PcaPsfDeterminerConfig(pexConfig.Config):
    spatialOrder = pexConfig.Field(
            "spatial order for PSF kernel creation", int, 2)

@pexConfig.registerConfigurable("pca", psfDeterminerRegistry)
class PcaPsfDeterminer(object):
    ConfigClass = PcaPsfDeterminerConfig
        # associate this Configurable class with its Config class for use
        # by the registry
    def __init__(self, config, schema=None):
    def determinePsf(self, exposure, psfCandidateList, metadata=None):

psfDeterminerRegistry.register("shapelet", ShapeletPsfDeterminer)

class MeasurePsfConfig(pexConfig.Config):
    psfDeterminer = measAlg.psfDeterminerRegistry.makeField("PSF determination algorithm", default="pca")