bug fixes for new local hyperdynamics reduced comm

    int nonzero = 0;

    m++;                           // placeholder for count of atoms

    for (i = first; i < last; i++) {

      if (maxstrain[i] == 0.0) continue;

      if (maxstrain_domain[i] == 0.0) continue;

      nonzero++;

      buf[m++] = ubuf(i-first).d;      // which atom is next

      buf[m++] = maxstrain_domain[i];  // value

   reverse communication invoked by a Fix with variable size data

   query fix for pack size to insure buf_send is big enough

   handshake sizes before each Irecv/Send to insure buf_recv is big enough

   this removes the if tests on sendnum and recvnum to make MPI calls,

     as in reverse_comm_fix(), b/c the caller may still want to

     exchange a message even if the send/recv swap has no atoms,

     e.g. a reduced count = 0 of atoms to send/recv, as in fix hyper/local

------------------------------------------------------------------------- */

void CommBrick::reverse_comm_fix_variable(Fix *fix)

      MPI_Sendrecv(&nsend,1,MPI_INT,recvproc[iswap],0,

                   &nrecv,1,MPI_INT,sendproc[iswap],0,world,

                   MPI_STATUS_IGNORE);

      if (sendnum[iswap]) {

      if (nrecv > maxrecv) grow_recv(nrecv);

      MPI_Irecv(buf_recv,maxrecv,MPI_DOUBLE,sendproc[iswap],0,

                world,&request);

      }

      if (recvnum[iswap])

      MPI_Send(buf_send,nsend,MPI_DOUBLE,recvproc[iswap],0,world);

      if (sendnum[iswap]) MPI_Wait(&request,MPI_STATUS_IGNORE);

      MPI_Wait(&request,MPI_STATUS_IGNORE);

      buf = buf_recv;

    } else buf = buf_send;