Add support for more windows to open window detection

[OTDose]

Hey, i'd like to have support for more "opened windows" sensors.

In our office we have setup automatic heating and there are two windows and two TRV's. But problem is the windows are both on one side but the TRV's are on different sides. So i cannot set one to each which still wouldn't end up working as intended. And if you would add such function. At the same time it would be great to have support also for more external temperatures and create an average temperature from that, because there is different temperature on each side.

In case you decide to add such function don't hesitate to contact me for testing it.

[dannykorpan]

As a workaround you can use a helper, add both door/window sensors and add the helper as the door/window sensor in better thermostat.

[folfy]

#879 and explained in the projects README, just use sensor groups (also can be created in the HA UI already)