Reads Second Wind C3 anemometer using the built-in ADC.
The anemometer output is a sine wave with a frequency proportional to cup rotational speeed.
The board's ADC cannot measure negative values, so the AC wave is clipped.
Non-zero analogRead() values are counted as pulses. A 20 millis delay is necessary to avoid mulitple counts that may occur on one pulse.
The task runs in parallel with the main loop, on the second core.