Minor improv in threads

Minor improv in threads

Let the user_bound for bounds given by S4U user (if any).

For the requested_cores, the bound is already configured when creating
the action at CpuCas01Action.

Also, the current implementation for threads use the same mechanism used
by VMs, for a N-thread action:
- priority/sharing_penalty: 1/N to increase the amount of CPU given to
action.
- individual bound: speed < N*C: to limit the action speed to the number
of cores.