Allow parameterisation to be done on different image to detection

[MatthewWhiting]

Often, it would be good to do the source detection on one image, but not the parameterisation. For instance, providing some deeper or combined or processed image to run the source-detection on, but then provide another image on which to make the measurements of source properties.

With the refactoring, can we set up the code so that it has these bits independent enough that this can be done? This would save having to save a mask or similar and re-running Duchamp to do the parameterisation.

[Cassandra]

I read your potsing and was jealous