Since VREF is an implementation detail, there shouldn't be a need to
include it in reports.
The ChannelState vref is removed along with it as its only use was to
save VREF measurements for later reporting.
Rewrite of PID according to https://hackmd.io/IACbwcOTSt6Adj3_F9bKuw PID implementation.
To migrate:
- TEC+/- pin polarity has to be reversed.
- Some saved settings might be wiped upon flashing of new firmware, back up settings before upgrade
- Min / Max integral parameters no longer exist
- kp, ki, kd will likely need to be retuned
The software has been tested on hardware with good temperature control stability.
Reviewed-on: M-Labs/thermostat#68
Co-authored-by: topquark12 <aw@m-labs.hk>
Co-committed-by: topquark12 <aw@m-labs.hk>
