- add preflight_check helper

 - this helps tease apart the various pieces of commander.

It uses the LP worker queue to periodically check for vehicle commands (30hz),
useful for several housekeeping tasks.
Currently the only task is temperature calibration.

Commands can be started via command line or via vehicle_command (from
Mavlink)

TODO: need to spec & extend the mavlink command.

This is necessary to allow more and better unit testing.

Restructuring of generic middleware support files, wrapping of the main ROS calls, skeletons for publishers / subscribers