Warn if attempting to apply function to a CFA image that isn't suitable for CFA images

Initially written for background extraction but made generic and applied to all functions that are generally unsuitable for use on CFA images.

Closes #1079 (as all the other parts of that issue are either addressed or covered elsewhere).

This is mostly low priority, however the addition of an override for the background extraction grid tolerance does apparently add some functionality for users with very bad gradients. I've set the milestone to 1.4.0: it could wait till after 1.2.0 but should probably be merged into 1.2.x when you're content that the warnings are applied to the correct set of GUI processes and commands.