Improve voltage calibration

[raimund-schluessler]

Things to improve on the spatial calibration:

• Use separate thread, so the main thread is not blocked
• Allow to set the min and max voltages and number of steps
• Create modal for settings these parameters
• Provide progress bar and allow to cancel
• Implement spatial calibration, allow to chose number of iterations

Using an iterative approach for the spatial calibration should increase its quality.

• 1. Do a "rough" calibration first with no prior knowledge about the position-voltage relation. Hence, large voltage ranges have to be used to really cover the whole camera sensor. This is the default atm.
• 2. Use the rough calibration to calculate voltages corresponding to the area of the camera sensor and calibrate again with these voltages. This way only voltages corresponding to a focus on the camera sensor will be used and the number of valid points increases leading to a better calibration.