porosityModels: Specification of name and dimensions of porosity coefficients is now optional

e.g.

    DarcyForchheimerCoeffs
    {
        d   (5e7 -1000 -1000);
        f   (0 0 0);

        coordinateSystem
        {
            type    cartesian;
            origin  (0 0 0);
            coordinateRotation
            {
                type    axesRotation;
                e1      (1 0 0);
                e2      (0 0 1);
            }
        }
    }