When using image plane priors, I think it would be nice to give the source positions on source plane

[lirui10590]

When I use image plane priors for the background source positions, it sees that the output parameter of the source position are on the image plane too. I think it would be nice to give the positions on source plane.

[ntessore]

cc @fabiobg83 @rbmetcalf

I think that the output parameter in the posterior should always match the prior input parameter, so I would be against replacing the image plane output with a source plane output. However, I think it would be possible to introduce additional derived parameter that contain the source plane values.

[lirui10590]

introducing additional derived parameter is a good way ,