[BHE] Add a new flow and temperature curve control condition and a comment for flow rate in 2U BHE benchmark
In this PR, a new flow and temperature curve control condition is added in the HeatTransportBHE process. By using this control condition, users could uses the flow rate and the inflow temperature curves as the boundary condition in time. The new feature is used in the benchmark "3D 2U BHE". Besides, a comment for the flow rate input in the 2U BHE model is added in the benchmark, which reminds the user the input flow rate value means the flow rate within each U-pipe. Detailed changes in this PR are list as followed.
- Added a new "TemperatureCurveFlowCurve" type in the flow and temperature control conditions.
- Added the description for the "TemperatureCurveFlowCurve" type in the BHE user-guide documentation.
- Changed the "flow_and_temperature_control" condition in "3D 2U BHE" benchmark from "TemperatureCurveConstantFlow" to "TemperatureCurveFlowCurve". In the flow rate curve, the value kept constant.
- Added the flow rate description in the documentation and a comment in the input prj file. Added tags documentation for the "TemperatureCurveFlowCurve" type.