- ABC history is now populated with previous outcome, so it won't
assume any plateau is the new low.
- Don't change baseline if it's close to the old one, to avoid flash wear
and display jumps.
- Reduced tick from 6 to 5 seconds, which is the MH-Z19's interval.
- Implement moving average for display to make it less jittery.
- Show ABC calculations and smoothing on serial output for debugging.
- Increase default mqtt failure maximum to 100 because restarts are
visually annoying.
- Moved some magic literals to named constants.
This one, with version number 0436, has a magic "I'm still booting" value
of 436 PPM instead of 410 PPM.
Interestingly, another sensor with version 0430 returns 430 for an instant,
but then goes to 410. During the short 430-phase the clamped CO2 value is
actually 0, so it doesn't require checking.
