Implement mixed boundary condition (Robin) in Generic module

[vidulejs]

Implemented mixed boundary condition (Robin) in Generic module. This allows to replicate, for example, CHT cases where there are Sink-Temperature and Heat-Transfer-Coefficient coupling data. I've been able to validate the implementation on the tutorial heat-exchanger (pictures to follow).

There are new operation options available to set the mixed boundary conditions when reading data from preCICE:

• ref-value
• ref-gradient
• value-fraction,
  which will set the respective fields in the OpenFOAM mixed BC.

To test the implementation standalone from the CHT module, I created a new tutorial case partitioned-heat-transfer-robin on my fork of the tutorials.

Here's the example preciceDict configuration for that case:

participant Left;

modules (generic);

interfaces
{
  Interface1
  {
    mesh              Left-Mesh;
    patches           (interface);
    
    readData
    (
      Sink-Temp
      {
        name        Temperature-Right;
        solver_name T;
        operation   ref-value;
      }
      HTC
      {
        name        Gradient-Right;
        solver_name T;
        operation   ref-gradient;
        flip-normal true;
      }
    );
    
    writeData
    (
      Write-Temp
      {
        name        Temperature-Left;
        solver_name T;
        operation   value;
      }
      Write-Grad
      {
        name        Gradient-Left;
        solver_name T;
        operation   surface-normal-gradient;
      }
    );
  };
};

TODO list:

• I updated the documentation in docs/
• I added a changelog entry in changelog-entries/ (create directory if missing)

[MakisH]

Looks good overall, some minor suggestions.

From our discussion: The robin test case would also be nice to have as a tutorial, with the new schema, to use as a test case.

Reply by @vidulejs: We don't have the complete support yet, as we are missing the conductivity calculation. In this case, it happens to work because the coefficient is 1. In that sense, adding this tutorial wouldbe misleading at the moment.

[MakisH]

We could add similar checks and error messages here: if not a supported operation for this BC type, throw an error.

[MakisH]

Maybe add a comment for the motivation for both checks here.

[MakisH]

Needs a changelog entry as well.