Add a proper nb_switches output

[mpoquet]

This information is quite hard to get a posteriori, but it can trivially be computed during the simulation.

[mpoquet]

Added in 0c5aea1. The _schedule.csv output file now contains:

• nb_machine_switches : The number of machine switches
• nb_grouped_switches : The number of requested grouped switches.

Invariant: 0 <= nb_grouped_switches <= nb_machine_switches