Controller for balacing agents (i.e. reservoirs and plants) pump.
This controller decides to turn on or off the agent pump depending on the current net demand of the
district if TRetDis > TSupDis cooling else heating  and the temperature differential
between the agent source temperature TSou and the agent inlet temperature TSouIn
adjusted by the offset dToff. In particular the pump turns on : if heating and TSou - TSouIn - dToff > 0
or if cooling and TSouIn - Tsou - dToff > 0. Then if the pump is turned on a PID controller,
by default used as P, controls the pump control input by using TSouOut as measurement and as setpoint
TSou - dToff for heating or TSou + dToff for cooling. dToff can be considered
the nominal value of the agent heat exchanger pinch point temperature difference.
Ettore Zanetti, David Blum, Michael Wetter.
Control development and sizing analysis for a 5th generation district heating and cooling network using Modelica.
Proc. of the 15th International Modelica Conference,
p. 23--32, Aachen, Germany, October 2023.