[sensor/camera] Handle more cleverly relations between objs of different scenes

Contrarily to what we can expect, several objects can have the same name
in the same scene. So we need to track the relation between the objects
of the different scene in a smarter way. As said in the code comment,
I'm not completely sure it is totally correct, but it largely improve the
situation, and I can't see any regression.

Should fix #509
[time] Update the timestamp from ms to s

Fix #498
[doc] Rewrote good part of the semantic camera documentation

Should make the changes introduced in #499 easier to understand.
[sensor] Fix semantic_camera example

Hi Séverin! :D
[sensor] Fix compound sensor, which was broken by timestamp addition

We previously test against "local_data" to be empty, which is never the
case nowadays, as it always contains at least a timestamp. Use a
specific flag to check for late initialization.
[ros] refactor pose publishers and add PoseWithCovarianceStamped

[ros] pose: only publish if data is marked as valid

if the sensor data does not have a valid field, always publish
[ros] read_pose: add PoseStampedReader
[ros] read_pose: also set position and orientation

as mathutils Vector and Quaternion respectively
[ros] pose: use functions instead of PosePublisherHelper class
[ros] Check if 'valid' is present instead of relying on exception handling

Reported by Pierrick
[ros] pose: check key in dict instead of try..catch