blockMesh: Added support for (<block> <face>) specification of patch faces

e.g. for the cavity tutorial the moving wall patch can be specified in
terms of the block vertices as before:

boundary
(
    movingWall
    {
        type wall;
        faces
        (
            (3 7 6 2)
        );
    }
    .
    .
    .

or the new specification of the face as block 0, block face 3:

boundary
(
    movingWall
    {
        type wall;
        faces
        (
            (0 3)
        );
    }