Clicpix2: replace lookup table for linearized threshold by analytical calculations

Analytical implementation for linearized threshold, assuming threshold_msb step to be 4.5mV and threshold_lsb step to be 0.5mV. Fine-tuned to avoid steps as much as possible.

Plots show result of threshold dac scan (car-board adc), before and after tuning. C3PD was powered, clicpix2 shutter not triggered during the scan. This has been done with the "testbeam" assembly. Before merging, the function needs probably more testing, also with the other available chips. Just want to start the discussion.

Initial scan (modulo factor 128):

After tuning (modulo factor 121):

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