Garage Door Sunset Monitor: If more than one door is opened, the threshold is double-counted

See https://github.com/nicjansma/smart-things/blob/master/smartapps/nicjansma/garage-door-sunset-monitor.src/garage-door-sunset-monitor.groovy#L191

If more than one door is opened, the threshold is multiplied.

So if you have a threshold of 10 minutes, and two doors are open, it'll only wait for 5 minutes before alerting.