Improved scheduling

Add the possibility to collect execution time stats for the scheduler with a simple ExecTime class. Avoid accumulating execution timing over multiple forward pass in mScheduling member variable (it is now reset at every pass).

Usage:

...
scheduler = aidge_core.SequentialScheduler(aidge_model)
exec_time = aidge_core.scheduling.ExecTime()

for i in range(NB_SAMPLES):
    scheduler.forward(True, [tensors[i])
    exec_time.update(scheduler.get_forward_scheduling())

exec_time.log("exectime")