Add support for MH-Z14 CO2 Module

[adoeller]

PWM or UART output

[ricki-z]

For the ESP8266 we need to use SoftwareSerial. But a second UART device is too much for the ESP8266. Our main goal is the PM measurement. So all other components should have an I2C interface. And there are CO2 modules with I2C interfaces. At the moment we try to integrate the Sensirion SCD30.

[str3yt]

If I would order a Sensirion SCD30 and flash beta firmware to test it - will the CO2 data show up on madavi.de (since there are only for sections WiFi, PM, T/RH/P and DNMS)? Should I better use OpenSenseMap? Or do I have to host my own API? Thanks :-)

EDIT: Sensor was delived, got it running with the current beta firmware, then I found out that madavi.de supports CO2! Just click "Dashboard" (Symbol consisting of 4 tiles on the left) - "Browse" and then select "CO2"
https://api-rrd.madavi.de/grafana/d/5Gzf52lGk/co2?orgId=1
Then enter the number of you sensor in the search field on top.