eliminate unnecessary srand calls

[alecjohnson]

Currently Particles3D::particle_repopulator contains code like this:

  srand (vct->getCartesian_rank()+1+ns+(int(MPI_Wtime()))%10000);
  if (vct->getXleft_neighbor() == MPI_PROC_NULL && bcPfaceXleft == 2)
  {
    [...]
  } 

I do not see any need to seed the random number generator more than once in the program, and I would rather eliminate these calls. Seeding the random number generator destroys the ability to get exactly reproducible results (except in the case that no data generated by calls to rand() is actually being used), and there should be a single point in the program which selects whether the seed is deterministic or comes from a time value that will be different with each invocation of the program.

Even if the srand calls are retained, I would change the code to:

  if (vct->getXleft_neighbor() == MPI_PROC_NULL && bcPfaceXleft == 2)
  {
    srand (vct->getCartesian_rank()+1+ns+(int(MPI_Wtime()))%10000);
    [...]
  }