SED STACKER - allow other mathematical operations on Stacks

[jbudynk]

[For a release after 2.1 ...]

E.g., subtracting a stacked SED from all or a selection of the SEDs in a Stack, which is useful for seeing which SEDs in the Stack are different from the majority of SEDs. [We would need to interpolate the stacked SED to the grid of the SEDs its being subtracted from.]

This could be useful in other parts of Iris, so we will want to plan this idea out a bit more...