added molecule option to compute group/group command

doc/src/Manual.txt

doc/src/Section_commands.txt

doc/src/compute_group_group.txt

examples/VISCOSITY/in.einstein.2d

examples/coreshell/log.9Nov16.coreshell.dsf.g++.1

examples/coreshell/log.9Nov16.coreshell.dsf.g++.4

potentials/Ge.tersoff

src/GRANULAR/fix_wall_gran_region.cpp

src/GRANULAR/pair_gran_hooke_history.cpp

src/GRANULAR/pair_gran_hooke_history.h

src/USER-MISC/dihedral_nharmonic.cpp

src/compute_group_group.cpp

src/compute_group_group.h

src/fix.cpp

src/pair_buck.cpp

src/version.h