Launch files that make working with TB3 in foxy a bit more convenient.
In part, these address the issue that the Turtlebot continues to obey
last cmd_vel
message forever, which can be awkward if a control node
crashes or exits while the robot is moving. the safe_cmd_vel
node
periodically checks to see if no other nodes are publishing to
cmd_vel
and if not, publishes a stop command.