Add support for analog to digital conversion ...

[nkolban]

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While the Pi does not have any in-built analog to digital conversion, there are sets of well known integrated circuits that can be wired into a Pi solution that provide ADC. This issue suggests that we choose some ADC ICs, describe some sample circuits and provide user selectable "mappings" to driver code that will drive those ICs to provide ADC. Thus should a sketch need analogRead() (see ... https://www.arduino.cc/en/Reference/AnalogRead) ... then the ADC IC would be driven and the result would come up through the analogRead() output.