[lammps-users] benchmarks for different potentials

FYI, added some new benchmarks to compare the relative
CPU cost of different potentials, as implemented in LAMMPS.

See the table of results here:
lammps.sandia.gov/bench.html#potentials

Thanks to various experts in several of the potentials
(e.g. the authors) who provided help of setting up
prototypical problems.

Steve