Logic for when to write output files needs improving

[chris-simpson]

When a recipe has been completed, there is logic to determine whether to write the output files to disk. This is meant to be decided based on whether the outputs are different from the inputs (to avoid writing when the display() primitive is run as a recipe). This logic is insufficient as, for example, it doesn't work on the GHOSTDR findApertures() primitive, which leaves the data alone but appends extensions.

One obviously can't keep full copies of the inputs in memory for comparison with the outputs but we need to do better. Maybe save the headers for comparison?