Add ability to abort calibration

[mprib]

Feeback from JOSS review:

It could be useful to add a button to stop camera calibration, volume calibration, and post-processing once they have started. Or just to make these buttons clickable a second time to abort. For example, this can be useful if we process long videos and realize we chose wrong parameters.

[mprib]

Just commenting here to note that implementing this will likely be a long-term goal and not a short term goal. Experience suggests to me that killing and restarting threads is a very easy way to create very bad memory leaks that are exceptionally difficult to track down.