Measure cumulative counts in Exposure Meter and put into header

[awhoward]

We want to know the cumulative counts in the exposure meter channels (I think these are 445-550 nm, 550-650 nm, 650-750 nm, and 750-870 nm, though we should check that these are the same channels used in @bpholden's realtime code that processes the EM images). We want these count values for the science and sky channels.

@bpholden, @jonzink -- please comment on what corrections, if any, to the EM flux measurements need to be made. We would like the EM counts to be the same no matter what read time is chosen. This is because the EM counts are used to predict the signal level in the main spectrometer.

The code should also gracefully pass if no EM frame is present.

[awhoward]

I think we'd also like these keywords to be read into Jump

[awhoward]

Possible keywords are:

EMSCI500
EMSCI600
EMSCI700
EMSCI800
EMSKY500
EMSKY600
EMSKY700
EMSKY800

[awhoward]

We should also add the signal measurements in the same units that @jonzink is using (average e- per Ang over some range of wavelengths, I think).

[awhoward]

I wrote some code to populate header keywords for cumulative exposure meter counts. See this PR: https://github.com/Keck-DataReductionPipelines/KPF-Pipeline/pull/734