Add an option to disable the 3% heuristic

[BenoitMorel]

By default, ParGenes assign twice more cores to the 3% jobs that have the highest number of taxa. This allows us to get rid of most of the "tails" (the few cores that finish much later than the other) and improves the load balance.

But in some cases, especially when the number of jobs is low, this heuristics might slow down the whole execution. Advanced users should have the possibility of disabling this heuristic. (and it will be useful to compare with/without the heuristic!)