Add battery measurement calibration procedure

[denis-stepanov]

It has been observed that two modules built to identical schematic do not exhibit the same behavior with regard to battery measurement. Static coefficients carefully tuned on one module could show very different values for another (like 40% instead of 90% for a fresh battery). Apparently, there is a need to introduce a calibration procedure for each newly assembled module, perhaps, using a separate sketch. The obtained coefficients could be either entered back into main sketch, or - more conveniently - stored in EEPROM (hence, replacing a ESP would require re-calibration). Still, some defaults should be kept with the code, so that some useful information could be deduced from measurements even if the calibration procedure has not been performed.