+ S4U
+ - New callbacks:
+ - s4u::onPlatformCreated: right before the simulation starts
+ - s4u::onSimulationEnd: right after the main simulation loop
+ - s4u::onTimeAdvance: right after a clock change (time is discrete)
+ - s4u::Host::onSpeedChange: when the pstate is changed, or when an
+ event from the availability_file changes the avail speed.
+ - Links are now usable from s4u
+