new fix flow/gauss command

doc/.gitignore

doc/html/Section_commands.html

doc/html/fix_flow_gauss.html

doc/src/Section_commands.txt

doc/src/fix_flow_gauss.txt

examples/USER/flow_gauss/README

examples/USER/flow_gauss/in.GD

src/USER-MISC/README

src/USER-MISC/fix_flow_gauss.cpp

src/USER-MISC/fix_flow_gauss.h

src/USER-REAXC/reaxc_ffield.cpp

src/USER-REAXC/reaxc_hydrogen_bonds.cpp

src/accelerator_kokkos.h

src/finish.cpp