Please split scattered light correction task from reduceExposure

The way that the scattered light code is currently implemented is as part of reduceExposure, but I think it needs to move to its own task.

• It logically belongs in its own task, writing the calexp
• It should really use the non-scattered light corrected fibre profiles to estimate the scattered light. This may not matter in practice
• If you process only a subset of the fibres (e.g. the ENGINEERING fibres) in reduceExposure, only those fibres are used in the scatted light correction

It is the last of these which means that I think we really need to make this change. The engineering fibres are interesting as they tell you how well the scattered light kernel has worked (once I commit the changes to make this actually possible; PIPE2D-1763)