ENH: support retention of original surface ids when sampling (related to issue #104)

For example,
    surfaces
    (
        helmet
        {
            type        sampledTriSurfaceMesh;
            surface     motorBike-passenger-helmet.obj;
            source      cells;
            keepIds     true;  <<-- NEW
        }
    );

    This will create an additional "Ids" field that can be used to sort
    or as a faceMap to recover the original face order.