add Exercise 3.0a (ROS1 intro/bridge)

[JeremyZoss]

Add initial exercise at start of Session 3. Introduces ROS1 command-line basics (roscore, rosrun, roslaunch). Does not address ROS1 syntax or compiling. Simple bridge exercise using existing pub/sub nodes. Nothing integrated into the overall "scanNplan" application, so can be easily removed later.

This exercise helps set the stage for later exercises: 3.3 requires using ROS1 to run MoveIt Setup Assistant, and 7.2 uses the bridge to run descartes motion planner.

[jdlangs]

Looks solid on my end. It would be nice to not make it seem like an attachment to the URDF exercise but there doesn't seem to be a great solution. I'll throw out a suggestion of labeling it a "special" exercise (S.0) and categorizing it separately. Maybe 7.2 also gets moved into that category too instead of just being labeled "advanced".

[JeremyZoss]

I'll merge this as-is, just to keep things moving along. We can renumber/reorg the exercises later if desired.