Allow sources to feed in different temperatures than the node temperature

[SimonRubenDrauz]

It is currently not possible to transfer a temperature to sources. Instead, it always refers to the temperature of the node.
The solution must be implemented in the core. A starting point can already be found in the code.

It is currently implemented in such a way that the mass flowing in from connected branches is the reference mass for the energy equation. This means that the sources cannot influence the node temperature. It is assumed that they feed in at node mixing temperature. This seems rather unrealistic. The correct approach would be a total mass balance across all connected components, taking into account the conservation of energy.
A first solution can be found in the code on develop. However, what is still missing, are the load derivatives what could affect convergence.