Method to reference channels for Issue #619

[kkashiva]

added method signature and implementation comments to DataFilter.java

Creating WIP draft pull request for the issue #619

Next: complete implementation for java package and then do the same for other language packages.

[Andrey1994]

Hi and thanks for working on it.

Java part looks fine.

You can add it to one of examples we already have, I do not think that we need a new one.
Downsampling should be ok - https://github.com/brainflow-dev/brainflow/blob/master/java_package/brainflow/src/main/java/brainflow/examples/Downsampling.java just keep in mind that this example also used as a code sample at BrainFlow Docs, so write some comments there for people who use it as a reference.

[Andrey1994]

no activiti in PR, closing