Flat Fielding Work

[wtgee]

There are a number of items that need to be completed in order for us to be capturing proper flat fields.

• [x] Take flats from pocs_shell. #736
• [x] Adjust predicted exposure time to account for morning. #729
• [x] Look into adding dithering code (from Huntsman). #735 #739
• [x] Adjust azimuth according to unit location (see https://github.com/panoptes/POCS/pull/729#discussion_r234473926 and https://github.com/AstroHuntsman/huntsman-pocs/issues/64)
• [x] Fix flat field count index number (it's incrementing for each camera - maybe just skipping first index on second camera?).
• [ ] Add testing of flat methods.
• [ ] Take flats automatically as part of state machine. #373
• [ ] Do something different with bad flats.
• [ ] Add back dark fields.
• [ ] Refactor and clean up flat-field method.
• [ ] Upload flats to storage bucket.
• [ ] Fix "Waiting for flat-field image" message.
• [ ] Do safety checks while taking flats. But make sure unit parks if not safe.

Note that for many of these things it makes sense to move the flat logic up to the pocs level, which would allow access to various utils (safety, waiting, etc).

This work is originally pulled from #541.

[wtgee]

I am currently taking flat fields on PAN001 via pocs_shell and with dithering. I have done this via a temporary branch that first merges #729, #735, and #736 as well as a few other edits to bring them all together. So far it is working well.

@jamessynge I can share this working merge branch if you would like to try testing in the next few days otherwise we can work toward getting all these Issues merged.