slow ADC inputs

slow ADC inputs

The fastest-repeated measurements are needed for PWM LEDs. OTOH,
things like buttons, battery voltage, ambient lights, etc. can be
read less frequently, and should be read in a deterministic time frame.
So we will measure PWMLED current in the free-running mode (as fast
as possible), and the other "slow" ADC inputs with each PATTERN_DIV-th
timer tick.