Define how the processors distributed after the domain decomposition.
The processor grid determines how the working load is distributed among processors.
If the particles fill the entire domain relatively uniformly, normally, there is no need to specify how to determine the processor grid. However, if not, the user can specify the processor grid for the sake of best efficiency.
nx * ny * nz has to be equal to the total required number of processors.
The processor grid is determined by the minimal surface area of sub-domain.