Out of sequence data on the imu raw data publisher

[Daniel Stonier]

Apparently causing problems for cartographer.

From Damon:

https://github.com/yujinrobot/kobuki/blob/kinetic/kobuki_node/src/library/slot_callbacks.cpp#L169

We dug into a bit before writing our flat_world_imu_node. It seems to us that a couple things can go wrong here:

• It's possible for the same IMU reading to be published more than once (see line 158).
• Jitter in the sensor timing may result in line 172 computing something that's older than the last published data.