Don't send PPMs for sensors that have become corrupted / poisoned

The resistive layer of a sensor can also be 'poisoned' by particulates in the air - we refer to this as a `corrupted` or poisoned sensor.

Sensor corruption is permanent. When the firmware detects a corrupt sensor, it should log it to the SDCard and stop sending data for that sensor (avoid other components handling corrupt data). It should also send a magic number to the mobile (`-2`?), so the mobile app can tell the user to repair or replace the device. The mechanism for detecting a corrupted sensor is to-be-worked-out during the calibration - possibly something along the lines of _‘the standard deviation of 10 sensor readings is fixed and not changing’_