THIS IS A PRE-RELEASE FOR TESTING IN A REAL ENVIRONMENT

As proposed by @garywg here #339, I add two entities to control the boiler starts:

1. an entity of type sensor counting in real time the number of devices which are heating (and that is marked as "controls the boiler"). If a VTherm have many underlyings, each underlying counts for one. I don't count the VTherm but the number of underlyings entities which are heating,
2. an entity of type number which holds the threshold of devices number heating under which we don't start the boiler.

It is working on my test environment but I need tests in real condition to be sure.