Switch CModel schema usage to version 1: FunctorKeys, '_' delimiter
Documentation improvements for CModel
Fix bad doc reference in SoftenedLinearPrior
Convert CModel to meas_base exceptions and method signatures.

Using MeasurementError also enabled a cleanup of the error-handling in
the determine*FitRegion methods, because we can now store which flag
to set in the exception instead of having an exception throw for one error
condition and a null return for the other.
Python plugins for CModel magnitudes.
Fix execution order for ShapeletPsfApprox plugin