ConductionElement variable heat transfer coefficient

[RaphaelGebhart]

Currently ThermofluidStream.Processes.ConductionElement only allows a constant (parameter) overall heat transfer coefficient U, however it might be beneficial to enable also variable heat transfer coefficients U (from input).

ThermofluidStream.Processes.Internal.PartialConductionElement would support this.

Actually this is already achievable by using Modelica.Thermal.HeatTransfer.Components.Convection, since its convective thermal conductance Gc is variable. To eliminate the thermal conductance k_par of the ConductionElement, one would need to set a large value for it, e.g. k_par = 1e9 W/K. I'm not sure if this may cause problems, but i assume it's not the best solution.

We could either enable U from input, k_par from input or both.