new USER-OMP package

Hi all - Axel has released a new USER-OMP
pacakge - see the 12Oct11 patch. This
has OpenMP versions of almost all pair
and dihedral styles. It's designed to allow
efficient shared-memory, threaded execution on
many cores of a multi-core node, which can
be more efficient than the default all-MPI mode
in some cases. Check out the doc/Section_accelerate.html
section of the manual for details.

Thanks to Axel for all the hard work that went into this!