The slot mechanism in afwTable currently uses compound fields to save the 3 slot types: flux, centroid, and shape. Since the new measurement framework uses a flattened representation in the SourceTable where these types are saved as multiple scalar fields, the slot mechanism need to be altered to handle this new table type.

1. An alternative to KeyTuple for storing the keys required by the slot
2. Fixup get(Centroid, Flux, Shape) in SourceRecord to use correct keys.
3. Fixup the single value getters (getX, getY, etc) to use the correct keys.
4. Persist slot info to fits correctly, based on table version.