Added a way to specify if the MPI operation used in global scans is commutative.

[asrivast28]

Problem: There was no way to specify if a custom operation used in global_scan* is noncommutative.
Solution: Added a parameter to the global_scan* functions (only the ones that accept a function as a custom operation) for specifying if the operation is commutative.

PS: This is possibly a hacky solution for the problem. In my opinion, a possibly better way to handle this is to add a variation of global_scan* which takes mxx::custom_op as an argument. The user can then create the custom_op for noncommutative operations and provide that as an argument to the global_scan* operations. I think this would be closer to the way MPI interface handles this.

I have gone with the least intrusive solution for now but can implement the aforementioned solution if it seems more sensible.

[asrivast28]

Added overloads for global_scan* functions as per @patflick review #23 (comment)