Use heatsource based on network supply temperature

[tolga-akan]

Use heatsource if asset max temperature is larger than or equal to network supply temperature for the case with network supply temperature has 1 temperature value:

• Modify heat_physics_mixing so that heat_out of heat_source will be zero if network supply temperature is greater than the max temperature of the asset.
• Create a new test. Place 2 heat source and set the cheap heat source maximum temperature lower than the network supply temperature. Show that optimizer uses only expensive heat source.

[tolga-akan]

@KobusVanRooyen Ready to review