Refactoring fan hysteresis

[ryan-summers]

This PR fixes #62 by refactoring fans to use a static (miniconf-configured) duty cycle whenever any RF channel is enabled.

TODO:

• Test on hardware

[jordens]

I suspect we can even do static "all channels off -> fans off", "any channel on -> fans at 20 %". We'd just have to measure steady state temp differential versus ambient under maximum load conditions. But I think it gets a bit more complicated with the HL version.
@gkasprow

[ryan-summers]

It's also worth noting that if we do decide to fix the fan speed to 20% if any channel is enabled, we still have the OverTemperature fault condition to protect against the channels getting too hot.

[ryan-summers]

@jordens Do we want to proceed with fixing to 20% now or keep the current 20->100% scaling implementation until further investigation can be done?

[jordens]

What about the following:

• Add a single miniconf configurable fan setting f32 (0..1), default 0.2
• If all channels are Off, fans are off
• If any channel is Powered, fans are set to that configurable value.
• Remove all other feedback/control.

[jordens]

Sorry for the persistent nagging, but the top level fan_speed resource irritates me.

[ryan-summers]

No worries, it irked me while writing it, but my brain moved on too quickly to fix it, thanks for pointing it out!