Remove node ros::Rate period auto computation and only read it from a node parameter

[rsiryani]

ROS should handle the node at least two time faster than the highest frequency output message.
This is mandatory to make sure two consecutive messages dont' get the same ROS time.

For now, it's computed based on device output messages configuration but the implementation is complex and not safe.
Moreover, the current implementation doesn't correctly handle IMU Short messages.

We should also make sure it's working correctly with High Performance INS that doesn't expose output message configuration.

As a result, I recommend setting this only from a node parameter and updating the documentation to properly explain this.

The default value should be set to 400 to support standard SBG INS that output data at 200 Hz.