[sensor] Add timestamp for sensors

timestamp corresponds to the time computed by the simulator
Moreover, in debug mode, the 'simulator_time' field corresponds to the
real absolute computer time.
[core] Read sensor time from the robot method gettime

It allows to have different robots with different 'bugged' clocks
(timezone, initial shift, slow drift, ...) and use the correct timestamp
tag for each robot.
[sensor] Add a dummy sensor clock
[mw/ros] Adapt ros interface to use the introduced timestamp