Connect the sensor to base shield A0 port, download program then open serial monitor. Waitting for 2
minutes (the sensor need to preheat), then testing will begin. We define 4 status, first is high
pollution(with force signal output), second is high pollution(without force signal), next is low pollution,
the last is air fresh. The four signals in program is "force","high_pollution","low_pollution" and
"air_fresh".