Added support for stacked images

[cmccully]

This PR adds support for images that were stacked at site. This observing mode was introduced to limit our exposure time so that the 0.4m cameras can self guide (using the short exposures) but only ship back a single image. The images are assumed to be 32bit integers and are only pixel by pixel added (no other processing is done going from a single frame to a stacked frame).

Nearly all of the processing happen at the readout level. Therefore only the bias frames and noise propagation need to be updated.