bluesky dark frames needs a variable number of dark frames

[ambarb]

Currently, the number of images requested for a "light" run does not change when collected to replaced expired darks.

Beamline needs to produce some data to determine the number of required dark images so that the result is statistically accurate. This is because the exposure time of the camera varies (0.01s - 5 s). For now, maybe 50 images are a good starting point, and anyone taking exposures longer than 1 or 2 seconds should switch to manual dark collection.

@danielballan @mrakitin @jklynch

[mrakitin]

xref #32

[ambarb]

I can do some of the lifting, but probably not all of the lifting.