Refactor image_masking to be more clear so that image masking per instrument can be actualized.

[emirkmo]

Image masking code is hard to understand and test, refactor the code to make the functions explain each step

• (initialize mask, check for finite, mask_non_linear, ensure there is a mask in the image, update image mask, etc.)

Goal: Change image masking to be more flexible per instrument, especially to deal with really crappy instruments/reductions sometimes used by @lgalbany

This issue should enable a more robust/custom per instrument masking.