[SMPI] Prohibit execution of negative flops

[SMPI] Prohibit execution of negative flops

This was often caused by smpi/host-speed:-1 because
in some cases, flops = duration * host-speed

I replaced one smpi/host-speed:-1 with smpi/simulate-computation:0
due to a brief conversation with Augustin Degomme today:

14:22 < degomme> host-speed -1 c'est une convention qu'on utilisait
14:22 < degomme> c'est synonyme de simulate_computation=0
14:22 < degomme> ou un truc du genre
14:22 < degomme> quand l'option n'existait pas .