new neigh_modify exclude option, other SNAP changes

doc/src/compute_voronoi_atom.txt

doc/src/neigh_modify.txt

examples/USER/misc/gauss-diel/in.gauss-diel

examples/USER/misc/gauss-diel/in.gauss-diel-cg

examples/USER/misc/gauss-diel/in.gauss-diel-split

examples/rigid/in.rigid.tnr

examples/snap/Ta06A.snap

examples/snap/W_2940_2017_2.snap

examples/snap/W_2940_2017_2.snapcoeff

examples/snap/W_2940_2017_2.snapparam

examples/snap/W_2940_2017_2_He_JW2013.snap

examples/snap/in.snap.Ta06A

examples/snap/in.snap.W.2940

examples/snap/in.snap.hybrid.WSNAP.HePair

potentials/Ta06A.snap

potentials/W_2940_2017_2.snap

potentials/W_2940_2017_2.snapcoeff

potentials/W_2940_2017_2.snapparam

potentials/W_2940_2017_2_He_JW2013.snap

src/GPU/README