Add option to read from internal ADC or from external vmux pad with multimeter

Currently all scripts read vmux values from external pad using multimeter. But we should add an option in all scripts (except for ADC calibration script) to read the vmux values from the internal ADC instead.

Depending on how we read the values, the analysis will change. We need to perform ground subtraction when reading values from the vmux pad, while we do not need to do this when we measure the internal ADC. Therefore we need to add a flag in the metadata so that we know if the vmux values are from the ADC or external pad, so that we can analyze data properly (see module-qc-analysis-tools#9 (closed))

Tagging @epianori , @theim for info