Fix histogram computation for expressions.
Fix histogram computation for expressions.

We appeared to have an optimization where we don't re-set parts of
the histogram if they were calculated previously.  However, the
assumption that the old histogram is valid is violated when
computing histograms for expressions; the underlying data has
changed, so the histogram must be thrown out and generated anew.

git-svn-id: 353be9ac11a1e711854a13efcc2fc17d9d6b83e7@1619 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok