porosityModel: The "<porosityModel>Coeffs" sub-dictionary is now optional

For example the porosity coefficients may now be specified thus:

porosity1
{
    type            DarcyForchheimer;

    cellZone        porosity;

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

    coordinateSystem
    {
        type    cartesian;
        origin  (0 0 0);
        coordinateRotation
        {
            type    axesRotation;
            e1      (0.70710678 0.70710678 0);
            e2      (0 0 1);
        }
    }
}

rather than

porosity1
{
    type            DarcyForchheimer;
    active          yes;
    cellZone        porosity;

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

        coordinateSystem
        {
            type    cartesian;
            origin  (0 0 0);
            coordinateRotation
            {
                type    axesRotation;
                e1      (0.70710678 0.70710678 0);
                e2      (0 0 1);
            }
        }
    }
}

support for which is maintained for backward compatibility.