Support ADC auto trigger modes.

[jfpoilpret]

Currently analog::AnalogInput allows ADC on-demand (just by calling sample()).

AVR MCU ADC circuitry offers more interesting features, in particular "auto trigger" which allows repeated sampling on an analog input pin.

It would be nice to support this feature and propose a callback notification when e.g. latest sample has changed.

This may require refactoring of analog namespace because auto-trigger mode can work with only one analog input and shall prevent on-demand sampling while active.

[jfpoilpret]

Remove planned milestone as it is not clear how important this enhancement can be.
I will replan it only when I get requests to do so.