Mat Object: 2 MPI processes
row 0: (0, 0)  (1, 1)  (2, 4)  (3, 5) 
row 1: (0, 2)  (1, 3)  (2, 6)  (3, 7) 
row 2: (0, 8)  (1, 9)  (2, 12)  (3, 13) 
row 3: (0, 10)  (1, 11)  (2, 14)  (3, 15) 
row 4:
row 5:
row 6:
row 7: