Pre-release - Capability to manage multiple heaters with one VThermostat

This pre-release implements the feature:

1. Capability to manage multiple heaters with one VThermostat - #67

You now have to possibility to specify more than one heater in the configuration. In that case, the cycle in the cycle of each heater, the device_power is the device power of each heater also (and not the sum of all heater power).

The algorithm now make its best to lower the power consumption at a specified time by shifting the starts of heating while keeping the overall calculated power.

WARNING: a lot of rework have been done for this feature, so don't apply this pre-release on critical environment.