changes to imbalance weight factors

doc/src/Manual.txt

doc/src/balance.txt

doc/src/dump_modify.txt

examples/body/in.body

src/angle.cpp

src/angle.h

src/atom.cpp

src/atom.h

src/atom_vec.h

src/bond.cpp

src/bond.h

src/comm_tiled.h

src/compute.cpp

src/compute.h

src/dihedral.cpp

src/dihedral.h

src/dump_custom.cpp

src/fix.cpp

src/fix.h

src/imbalance_group.cpp