Method for MPAS parallelization (nCellsPerPET)

[weather4evr]

To figure out the MPAS parallelization, this is used in model_grid.F90:

nCellsPerPET = ceiling(real(nCells)/real(npets))
cell_start = localpet*nCellsPerPET+1
cell_end = min(localpet*nCellsPerPET+nCellsPerPET,nCells)
nCellsPerPET = cell_end - cell_start + 1

But, this will sometimes lead to nCellsPerPET < 0 for certain combinations of grid sizes and npets, probably because use of "ceiling" in the above. This algorithm is arguably safer:

call para_range(1, nCells, npets, localpet, cell_start, cell_end)
nCellsPerPET = cell_end - cell_start + 1

subroutine para_range(n1, n2, nprocs, irank, ista, iend)

  integer, intent(in) :: n1, n2, nprocs, irank
  integer, intent(out) :: ista, iend

  integer :: iwork1, iwork2

  iwork1 = (n2 - n1 + 1) / nprocs
  iwork2 = mod(n2 - n1 + 1, nprocs)
  ista = irank * iwork1 + n1 + min(irank, iwork2)
  iend = ista + iwork1 - 1
  if (iwork2 > irank) iend = iend + 1
  return
end subroutine para_range

`

[LarissaReames-NOAA]

This was fixed using your suggested solution in PR #18