public short asShort() {
        return this.mask;
    }
Converts one {@link RollUpBitMask} object into another using the passed
map of new tag positions to old tag positions. If an old tag position has
no corresponding new position, then that tag cannot be rolled up.

@param newToOldFieldPositionMap Map containing keys which are the positions of all the new
                                tags and values which are the old tag position that the new
                                tag position corresponds to.
@return A new {@link RollUpBitMask} object
stroom-statistics/stroom-statistics-impl-hbase/src/main/java/stroom/statistics/impl/hbase/rollup/RollUpBitMask.java