Merge branch 'add-pair-zhou' of https://github.com/tonnamb/lammps into pull-417

doc/src/Eqs/pair_zhou.jpg

doc/src/Eqs/pair_zhou.tex

doc/src/Section_commands.txt

doc/src/pair_zhou.txt

doc/src/pairs.txt

examples/USER/misc/zhou/Ag_O1X5.5_O2X0.55.eam.fs

examples/USER/misc/zhou/README

examples/USER/misc/zhou/system.data

examples/USER/misc/zhou/system.in

src/USER-MISC/README

src/USER-MISC/pair_zhou.cpp

src/USER-MISC/pair_zhou.h