Fix subtle sizing error.
Fix subtle sizing error.

Using the wrong type in sizeof.  Doesn't matter much, because
sizeof(double) == sizeof(UINT64).

git-svn-id: 353be9ac11a1e711854a13efcc2fc17d9d6b83e7@1358 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok