Issue39 ideal heater cooler

This pull request is to add a heater and cooler with temperature as an input signal.

It is a modification of the model developed by @nytschgeusen and @thorade that avoids a state event at zero mass flow rate.

The model also has fewer calculations as it does not required to compute first `Q_flow` and then add `Q_flow` to the fluid stream. This approach was also avoided because the regularization used in the energy balance (that is needed to compute `dh=Q_flow/m_flow` while avoiding issues due to numerical noise) would lead to the situation that the leaving water temperature is not maintained for very small flow rate. This could potentially propogate large errors into control loops. Therefore, the implementation was redone without using a fluid volume.