Generate noChoicesSet convenience method for Sets

[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 noChoicesSet()
    {
        return 0 == buffer.getByte(offset);
    }

[mjpt777]

This could be a useful feature. Would make sense to call it something more like isEmpty() and it also needs to include taking care of the null value when optional.

[mjpt777]

Please run the full build to ensure it passes.

[JerryShea]

Woops - fixed now.