add option to pair_modify to explicitly turn off F dot r

pair_modify keyword values ... :pre

one or more keyword/value pairs may be listed :ulb,l

keyword = {pair} or {shift} or {mix} or {table} or {table/disp} or {tabinner} or {tabinner/disp} or {tail} or {compute} :l

keyword = {pair} or {shift} or {mix} or {table} or {table/disp} or {tabinner}

or {tabinner/disp} or {tail} or {compute} or {nofdotr} :l

  {pair} values = sub-style N {special} which wt1 wt2 wt3

               or sub-style N {compute/tally} flag

    sub-style = sub-style of "pair hybrid"_pair_hybrid.html

  {tabinner/disp} value = cutoff

    cutoff = inner cutoff at which to begin table (distance units)

  {tail} value = {yes} or {no}

  {compute} value = {yes} or {no} :pre

  {compute} value = {yes} or {no}

  {nofdotr} :pre

:ule

[Examples:]

"kspace_style"_kspace_style.html command requires a Kspace-compatible

pair style be defined.

The {nofdotr} keyword allows to disable an optimization that computes

the global stress tensor from the total forces and atom positions rather

than from summing forces between individual pairs of atoms.

:line

The {special} keyword allows to override the 1-2, 1-3, and 1-4

      else if (strcmp(arg[iarg+1],"no") == 0) compute_flag = 0;

      else error->all(FLERR,"Illegal pair_modify command");

      iarg += 2;

    } else if (strcmp(arg[iarg],"nofdotr") == 0) {

      if (iarg+2 > narg) error->all(FLERR,"Illegal pair_modify command");

      no_virial_fdotr_compute = 1;

      ++iarg;

    } else error->all(FLERR,"Illegal pair_modify command");

  }

}