Updated the virtual warp kernel of the SSSP hybrid algorithm

Updated the virtual warp kernel of the SSSP hybrid algorithm so that the virtual warp width is never larger than the hardware warp size.
I would phrase it as follows:
Note that it is not possible to use a virtual warp width longer than the hardware warp width. This is because a vertex may switch from inactive to active state (maintained by the updated array) during the execution of a round.  This may lead to the situation where the threads of a virtual warp, which are all supposed to process the neighbours of a vertex, evaluate the vertex's active state differently, and hence part of the neighbours of that vertex will not get processed.