Proposal: replace ROS msg/srv files with a submodule which pulls in the relevant messages

[ssnover]

Currently, this repo copies a number of msg and srv files which are available in common_msgs or other ROS repositories. Additionally, some messages like diagnostics_msgs are not included but are necessary to build. These packages are available on systems which have a ROS workspace set up, but it'd be great for this repository to be self-sufficient to be easier to work on.

This could be managed by pulling in relevant repositories as submodules. I believe the necessary ones are:

• https://github.com/ros/common_msgs
• https://github.com/ros/ros_tutorials
• https://github.com/ros/ros_comm_msgs

Though there may be more (can't tell if benchmark_msgs is a standard msg package for example).