[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