Create samples showing how to use each built-in aggregation

[mtopolnik]

From @jerrinot on November 15, 2017 13:37

It took me a while to figure out how to use the built-in aggregations. We have code-samples for e.g. counting but:

1. There is too much noise. I don't want to deal with Shakespeare, splitting words and lines. All I want to see how to use averaging(). As simple example as possible. With minimum noisy.
2. We do not have any example showing how to use an aggregation which does not use key-based classification. e.g. averaging() (as opposed to counting()`

I am not sure if we need to have this as a code-sample. The current samples tries to create something resembling a real-word use-case. Even a simple junit integration test in the Jet project itself would help me a lot.

Copied from original issue: hazelcast/hazelcast-jet#599

[mtopolnik]

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