The use of is_sorted in Benchmarker.h was ambiguous

Benchmarker provides its own implementation of is_sorted since this method was not introduced until C++11 and not all compilers necessarily support it. However, for those that did, the system is_sorted conflicted with the provided is_sorted. To get around the problem, specify the full namespace of the is_sorted being used (which is standard practice in VTK-m anyway).