Massive PlanOperation refactoring

* move to namespace hyrise::access
* split into ParallelizablePlanOperation and PlanOperation
  * appropriate subclassing of existing classes
* removed unused PlanOperation API elements
* changed error reporting mechanism to not propagate through ops, rather just talk to response
* added notion of .json tests that may expectedly fail (xfail)
* clean up situation where failing plans may report as successful in tests

Sorry for the large commit. The heat of the refactoring moment. ;/
Merge pull request #137 from bastih/feature/clean_up_planoperation

PlanOperation refactoring lands