fix of seed + gen cost permutation + pv to pq

[albanpuech]

• fixed bug with random seeds being the same on all processes on some clusters. Now the seed is different for each process
• no permutation of constant gen costs (this led to dispatches with 0 total cost when the costs of generators that have 0 cost got assigned to large generators)
• Auto PV→PQ conversion: deactivate_gens() now automatically converts PV buses to PQ when all generators are deactivated

[albanpuech]

reviewed