Add ublox_msg_filters package with exact time synchronization construct

[kmhallen]

• Add ublox_msg_filters package to time synchronize multiple messages with iTOW to get a single callback
• Rename iTOW field of NavRELPOSNED messages to be the same as all other Nav messages

I wasn't sure about the license when adding a new package, so I copied the license header from other source code in this repository

[kmhallen]

I plan to provide something similar for the ROS2 branch in the next couple months

[versatran01]

Thanks for the PR. I had a quick look and have a few questions.

1. Why do we need a special message filter? Can't we just use the ros message filter?
2. The exact_time.h has the wrong license header if it is ported from the original one. https://github.com/ros/ros_comm/blob/noetic-devel/utilities/message_filters/include/message_filters/sync_policies/exact_time.h
3. The cmakelist of this package sets cxx standard to 17. This is different from the rest of the package. However, I think it's time we all move to c++17 so maybe we can do a synchronized change if this get's merged (unless you need c++17 features).

[versatran01]

Regarding the license, this package is missing a LICENSE file but has license header in some files.
I think we could add a permissive one like Apache-v2 and make it clear. @meyerj

[kmhallen]

It is a port of that filter. The message_filters one uses the header/stamp field that is a ros::Time type. The ublox messages don't have a ROS time stamp or a std_msgs/Header.

Additionally, a user is most likely interested in all ublox messages that were created based on the same measurement update (which is specified by iTOW). If a ros::Time stamp at message reception was used, each timestamp would be a little different.

[kmhallen]

The cmake could be changed to be more similar to the existing packages.

I'll defer to someone else's judgement on what to put in the license headers.

[versatran01]

Thanks for the explanation, that makes sense.

For the license on exact_time.h, I'm almost certain you need to keep the original willow garage one.
As for the rest, if we agree on a repo-wise license then you don't need to put in anything.

[kmhallen]

Okay, I'll use the Willow Garage one in the header and remove the license in the example cpp.

Should I keep set(CMAKE_CXX_STANDARD 17) or remove it?

[versatran01]

Are you using any 17 features by any chance? If not I suggest you change it to 11 to be compatible with the rest of the packages.
Later I can do a full CMake update to default to 17 if no one objects.

[kmhallen]

I'd prefer to remove it instead of specifying 11. The message_filters version doesn't specify, and I don't think it's using anything added in 11 or 17:
https://github.com/ros/ros_comm/blob/noetic-devel/utilities/message_filters/CMakeLists.txt

[versatran01]

Sounds good.