Fix wrong grid coord when coordinate at edge of grid

When the global coordinate was in the far edge of the grid, the returned
grid coordinate would be one more than the maximum in at least one
dimension resulting in comm.Get_cart_rank failing.

gprMax/grid/mpi_grid.py

@@ -116,14 +116,17 @@ class MPIGrid(FDTDGrid):
     def is_coordinator(self) -> bool:
         return self.rank == self.COORDINATOR_RANK
 
-    def get_rank_from_coordinate(self, coord: npt.NDArray) -> int:
+    def get_grid_coord_from_coordinate(self, coord: npt.NDArray) -> npt.NDArray[np.intc]:
         step_size = self.global_size // self.mpi_tasks
         overflow = self.global_size % self.mpi_tasks
-        grid_coord = np.where(
+        return np.where(
             (step_size + 1) * overflow >= coord,
             coord // (step_size + 1),
-            (coord - overflow) // np.maximum(step_size, 1),
+            np.minimum((coord - overflow) // np.maximum(step_size, 1), self.mpi_tasks - 1),
         )
+
+    def get_rank_from_coordinate(self, coord: npt.NDArray) -> int:
+        grid_coord = self.get_grid_coord_from_coordinate(coord)
         return self.comm.Get_cart_rank(grid_coord.tolist())
 
     def global_to_local_coordinate(self, global_coord: npt.NDArray) -> npt.NDArray: