Add HeatMeter as sensor

[JAG-1337]

When validating systems of components or heating networks, I would like to have a simple heat meter for measurement of thermal energy.

The implementation inherits from PartialDynamicFlowSensor and adds a temperature input T_other to finally calculate

Q_flow = port_a.m_flow * Medium.cp_const * (T - T_other);

with T being the temperature of the fluid passing through that sensor.

[mwetter]

That would be a good addition. Can you please make an implementation and pull request to the branch issue1831_heatMeter.