fvOptions/constraints/fixedValueConstraint: Replacement for the nonsensical ExplicitSetValue

Description
    Constrain the field values within a specified region.

    For example to set the turbulence properties within a porous region:
    \verbatim
    porosityTurbulence
    {
        type            scalarFixedValueConstraint;
        active          yes;

        scalarFixedValueConstraintCoeffs
        {
            selectionMode   cellZone;
            cellZone        porosity;
            fieldValues
            {
                k           30.7;
                epsilon     1.5;
            }
        }
    }
    \endverbatim

See tutorials/compressible/rhoSimpleFoam/angledDuctExplicitFixedCoeff
constant/fvOptions for an example of this fvOption in action.