Merge branch 'merge-pull-150' into lammps-icms

From Anders Hafreager:

As discussed today, an optimized vashishta implementation with 2
optimizations. Both optimizations increases performance by a total
factor 3.

1. tabulated 2-body forces. Simulation time (in.sio2 with 10k timesteps)

went from 155.8 secs to 80.51 secs. See attached .zip-file for output.

2. building a new short range 3-body neighbor list when looping through

2-body neighborlist. Since 3-body cutoff is 2.6Å and 2-body is 10Å for
SiO2, we can skip many 3-body triplets that are out of cutoff.
Simulation time went from 80.51 secs to 51.71 secs.

I also added a pair_modify table yes/no [tableSize] command. USER-OMP
version is also updated.

Additional optimizations, cleanups, and documentation by akohlmey@gmail.com