Decoder generated for a Set does not provide a convenience method to determine if no choices have been set

[JerryShea]

The decoder generated for a Set does not provide a convenience method to determine if nothing has been set. If we look at sbe_samples and Car's OptionalExtras, the only way to determine if no OptionalExtras
have been set is to call

    notSet = ! (sunRoof() || sportsPack() || cruiseControl());

And this code is fragile and inefficient. This PR modifies SbeTool to generate a new method in OptionalExtrasDecoder

    public boolean isEmpty()
    {
        return 0 == buffer.getByte(offset);
    }

PR here: #575