Add voltage calibration routine to API

The high voltage output may be measured via an on-board voltage divider using the ADC (see [this notebook][1]).

Add a calibration routine (based on [the code in the notebook][1]) to calibrate the high voltage output based on on-board measurements.

[1]: https://github.com/wheeler-microfluidics/dropbot-dx/blob/master/dropbot_dx/notebooks/calibrate_hv.ipynb