Update to make URDF usable in ROS2

[RMichaelSwan]

Fixed a number of issues with existing URDFs such as:

• Relative paths. Replaced with package relative paths (package:// instead of ./). Not sure if this would cause issues with other tools, but it does work with rviz2, robot_state_publisher, and joint_state_publisher in the context of a ROS2 workspace (and should work with ROS1 also).
• Incorrect/bad limits (replaced huge numbers with more sensible limits, though not verified with actuals).
• Incorrect joint type for drive joints (wheels, propellers). Changed from revolute to continuous.
• Tabs are not able to be ingested by robot_state_publisher, replaced with spaces.
• Colons inside comments cause issues with robot_state_publisher, replaced with dashes.

Also converted into ROS2 compatible package and provided a few examples for launching with rviz2.