Enable External ADC

[ChitoKim]

mahjongAsst measures RLC values by ADC readings. Naturally, its accuracy, especially for resistance measurements, depends heavily upon performance of the internal ADC. For example, I recently tried AMOS_MONSTER thing on ESP32, but the results were terrible. So mahjongAsst should offer an option to use an external ADC. ADS1X15 library looks cool. https://github.com/RobTillaart/ADS1X15