Concatenate method for DataPipeline class

[am831]

What does this PR do? Please describe:
Implements concat method in the DataPipeline class which concatenates two data pipelines.

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• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
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[cbalioglu]

Thanks for your first PR! My main feedback/question is why we need to have two lists of pipelines. I think the ideal API should look like .concat(pipelines: Sequence[DataPipeline]) (in C++ .concat(std::vector<data_pipeline> pipelines), so I can't see the reason for the second "pipelines2".

Looks like there are also some linting and build issues that you can check at the bottom of the PR conversation pane.

[cbalioglu]

Neat! Looks good to me. Please make sure to annotate your python test class TestConcatOp as I mentioned in our chat. There is also a subtle bug in the constructor initializer, but otherwise looks good to me. Let me know once you addressed the two points and I can stamp it right away.

[cbalioglu]

LGTM!