Fix OMP with Visual Studio.
Fix OMP with Visual Studio.

VS only supports OMP 2.0.  "collapse" is a 3.0 feature, and 2.0 had
some (completely absurd) requirement that loop indices be *signed*
types.


git-svn-id: 353be9ac11a1e711854a13efcc2fc17d9d6b83e7@1929 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok