Integer/float divide by zero by MPI_set_view when compiling with debug flags

[andyandreolli]

It could be that the below bug arises as a consequence of npy only; for sure, a the value of npy is one of the causes.

When compiling with debug flags, usually no problem arises with low values of npy. For high values of npy, one of the two situations arises:

1. the program is unable to terminate, as it is blocked on MPI_write;
2. the program terminates, the output seems correct, but if one uses debug flags for compilation, it can be seen that MPI_set_view triggers a float divide by zero.

It can be that issue no. 1 is associated to an integer divide by zero, whereas the second might be associated to a float divide by zero. For sure, in both cases MPI_set_view is trying to divide by zero.

It also seems (from a limited number of attempts), that case 1 (program unable to write and terminate) happens when processes have a different number of y points. Attention: the number of points held by every process includes ghost and halo cells; it can be found as nyN-ny0+5.