__threadfence() based single kernel reduction added (but not used)
__threadfence() based single kernel reduction added (but not used)

Initial performance tests indicate that there is no performance benefit
to using a single kernel reduction. Leaving the code comitted, but not used,
to facilitate further testing.

git-svn-id: f4d3f11dd30626d3fa67da974003c91fff19c220@2989 fa922fa7-2fde-0310-acd8-f43f465a7996

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue