BUG-509: Move DataTree concepts into separate package

Create a new package for the data tree and related concepts to clean out
interactions. Also create and document the API and hide implementation
classes from the user itself.

Change-Id: I69f2a9b7bcac863b531f46f669bfd2fcfcd5b743
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-509: Move DataTree concepts into separate package"