Auto balance point for heatpumps

by Daniel Born: Instead of relying on pre-defined and possibly arbitrary setting for minimum exterior heatpump temperature, automatically figure out the balance point for the heatpump and switch to auxiliary heat when it is necessary.

Also allow combining heat output from heatpump with first stage auxiliary heat for more economical heating compared to second stage aux heat only.

Another nice feature, automatic forward and reverse staging of different heating sources based on changing outside temperature.