More strip heat control options

[jmaguire1]

Right now we use a pretty basic control of the strip heat based on a cut in temperature. I'd like to add some additional options:

• Strip heat turns on only when space temperature is x degrees off of setpoint.
• Strip heat turns on when you can't reach setpoint after x minutes of running the HP.

This ties into #14 pretty directly. We'll still want to update the performance map and cold climate capacity retention, but backup strip heat control has a huge impact on cold climate performance as well.

[apoerschke]

Here is guidance from ecobee on how their thermostat handles temperature offset and runtime strip heat operation.
https://support.ecobee.com/s/articles/Threshold-settings-for-ecobee-thermostats

[apoerschke]

I was seeing abnormal behavior during low temps / HP lockout where the ER setpoint was immediatly met.

I've added logic to recognize when the HP is locked out and to have the ER follow the standard setpoint / deadbands.

Will add this to the branch.

[jmaguire1]

Awesome, thanks Andrew! Really excited to have you engaged and contributing here!