- use pragma once guard in FailureDetector.hpp
- send Commander parent to ModuleParams and remove update_params() method
- simplify attitude checks
- FailureDetector::get_status() (previously named "get") is now a constant method that returns a constant reference

FailureDetector - New class, first working simple implementation. Outputs PX4_ERR messages if roll or pitch is exceeded.

- Move publishing the telemetry status from the IridiumSBD driver to the mavlink instance
- In the commander use the iridiumsbd_status message for heartbeat in case of a high latency link
- Move positive acknowledge to the mavlink instance
- Add a failed acknowledge in the commander if no high latency link exists

 - 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