Calibration fails when a shutter isn't available

[talonchandler]

Yuming from the Elicieri group had trouble calibrating his LCs because of a poor black level estimate. We traced this problem to a his lack of a shutter. We found a manual workaround: turn off the light source during black level estimation, then turn it back on for the rest of the calibration.

I think a more complete solution will include:

• a warning to the user if a shutter isn't available
• a GUI element that lets the user input a black level by hand. This should be a pretty stable value, so I expect this to be a good solution.

[ieivanov]

I like this, I can implement it together with #111