[core] Create local_data with recursive definitions of _data_fields
[doc] Don't store a reference on attribute, but a copy of it.

Otherwise, you will update stuff from mother class, and bad things
happen
[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] Prepare ros interface to use the introduced timestamp

The code is not yet activated, because it requires fixing the
computation of timestamp, and requires testing at the ROS level.