[HeatTransportBHE] PowerCurveFlowCurve function

Added PowerCurveFlowCurve function to flow_and_temperature_control in HeatTransportBHE process. It allows you to define both a power curve and a flow rate curve which is used for the simulation. It is initially used for comparing simulation reuslts between coupled simulations between OGS and Modelica (surface building simulations, see also: server communication feature) and "simple" OGS simulations where we input the power and flow rate curve which is computed by an independent Modelica simulation in advance.

Note: I also made a test case for this, but since all the other flow and temperature control options don't have any test cases I didn't add it. If wished I can add it to the merge request.

I updated the web content documentation and also added the documentation for BuildingPowerCurveConstantFlow option which I found in the source code.

1. Feature description was added to the changelog
2. Any new feature or behavior change was documented?