Remove unnecessary include of PlanOperation
Small cleanup in HashJoinProbe
Move to abstract resources as operational data

We remove two distinct data structures for specific types and
replace this structure with a more generic one. The old interface
stays intact for AbstractTables and AbstractHashtables.

Further we clean up some details in PlanOperation and add a
set of tests for OperationData's behavior.
Merge pull request #35 from bastih/feature/abstract_intermediates

Move to abstract intermediates, fixes #19