Unify buzzer, leds, relays, Neopixels, and other "Outputs" stuff, in CO2_Gadget_Outputs.h.

[melkati]

          I already moved BUZZER_PIN to GPIO 02

I will leave:

To finish and before publishing into the release, unify it with the led, relays, Neopixels, and other "Outputs" stuff, in order to remove the code that is scattered everywhere, and rename it as CO2_Gadget_Outputs.h.

For after the release. I don't want to delay it any longer just for this reason (it would take a short time to do it but it involves a lot of testing afterwards)...

So I think it's ready for release...

Originally posted by @melkati in #154 (comment)