Preprocessing steps

[guynir42]

Refactor the way we request certain preprocessing steps to be done, e.g., applying flat or bias or fringe to certain images.

• allows instruments to define a list of what steps they can do, and what steps they already done
• user requests a set of steps to be done, either by pipeline or by the thing that provided the exposures.
• allow skipping certain steps for specific filters.

[rknop]

This looks good.

It occurs to me that the code assumes that DECam images always come in raw, and that's how we're usually doing it. But you can pull in reduced DECam images. When you do that, how do you specify that the steps_done field should be different from what's in the DECam class?

[guynir42]

I think the original idea would be to use a separate Instrument class for processed exposures.