Add guided capture of bias, dark, and flat frames for image stacking

[jaredkzr]

See this page for details.

• Bias: place cap on lens, take multiple shortest exposures possible, used to cancel out inherent sensor noise

• Dark: place cap on lens, take multiple at same exposure as brights, used to cancel out thermal noise

• Flat: point optical assembly at bright white source, expose to see dust/defects and take multiple exposures, used to correct defects in optical train

[jaredkzr]

Need to close #4 first so that these calibration frames are actually applicable