Completed initial implementation of parallel sort.  It uses an in-place ...

...merge sort implemented with parallel block exchange.  The time limit isn't set up yet, but all the parallel algorithms are cancellable.
@bsoetaer 
.. is not equivalent to ...
Is this the intent? that (0,-1,length) will return a range with max = length-1 but (0, length-1, length) will return a range with max = length-2? seems a bit odd to me if that is the intent.