Merge pull request #1089 from lammps/spread

doc/src/Commands_compute.txt

doc/src/Howto_chunk.txt

doc/src/compute.txt

doc/src/compute_chunk_atom.txt

doc/src/compute_chunk_spread_atom.txt

doc/src/compute_reduce.txt

doc/src/compute_reduce_chunk.txt

doc/src/computes.txt

doc/src/lammps.book

src/compute_chunk_spread_atom.cpp

src/compute_chunk_spread_atom.h

src/compute_reduce.cpp

src/compute_reduce_chunk.cpp

src/compute_reduce_chunk.h

src/fix.cpp

src/fix_ave_chunk.cpp