getVelocityDistribution should make one MPI_Allreduce call

[alecjohnson]

Particles3Dcomm::getVelocityDistribution() sums particle velocity distributions using:

  for (int i = 0; i < nBins; i++) {
    localN = f[i];
    MPI_Allreduce(&localN, &totalN, 1, MPI_LONG_LONG, MPI_SUM, MPI_COMM_WORLD);
    f[i] = totalN;
  } 

This generates thousands of all-reduce calls, which is horribly inefficient and will be unacceptable on DEEP's Booster. These lines can be replaced with a single call:

  MPI_Allreduce(MPI_IN_PLACE, f, nBins, MPI_LONG_LONG, MPI_SUM, MPI_COMM_WORLD);