providing accumulator example as proposed in FLINK-828

- shows how to build a custom accumulator (for vectors)
- employ that accumulator for obtaining filter statistics
Can you change the code that it does not require any parameters at all? (The user still has to figure out how to pass parameters to the program)
So if the user does not specify any arguments, we use `getExampleInputTuples()` to get some input data. We print the result using dataSet.print() to stdout.