add functionality to record of best_costs in file

Appli-iso3dfd/ACO.py

@@ -124,6 +124,9 @@ def ACO(Me, NbP, comm, alpha, rho, Q, nb_ants, tau_0, n_iter, n1_max=256, n2_max
         if best_cost < cost_opti:
             cost_opti = best_cost
             path_opti = best_p
+        if Me == 0:
+            with open("cost_opti.txt", 'a') as file:
+                file.write(f"{cost_opti}\n")
 
     n1 = 256 * (2**(path_opti[0]-1))
     n2 = 256 * (2**(path_opti[1]-1))