Exception when a query requires more execution nodes than available
Added basic implementations of Filter, FlatMap, and Difference
first implementation of the GroupBy operator
groupBy on sorted input (incomplete op.peek() solution)

in this version peek() only does something sensible on the GateReaderOperator,
not on other operators --> assumes that multiple operators aren't compactified
into a single task

a better implementation could entail separating the peek/next behavior from the
operator logic: e.g. having a dedicated op.nextImpl (implementing the operator logic) methods that call a basic op.next (which handles the peek/next logic)
groupBy on sorted input (tuple stream peeking done in groupBy)

this way not all operators have to implement peek() / the operator
interface remains small and easy..
 inadequate fix for in process buffer freeing at TASK_STATE_FINISHED