Feature/piksi multi cpp

[rikba]

A first shot for a cpp ROS driver as a response to the performance issues with the python driver #98 .

As of now this implementation includes

• A device base class that contains factory methods to create all possible piksi devices (USB, Ethernet, Serial).
• A USB implementation that automatically detects attached Piksis and open, reads and closes the ports.
• A ROS interface that owns all devices and creates SBP message callbacks
• A ROS node managing opening, reading and closing of all devices.

This minimum working implementation is merely complete and a starting point for design discussions and further implementations, such that additional interfaces, message callbacks and UDP broadcasting.

Currently we use <3% of a i7-7820HQ core which is subject to further optimization on how the polling can be done.

[michaelpantic]

Awesome, really nice!

See comment about publisher architecture (this could simplify our life later), but we can also merge it like this and figure this out in the next PR.

[michaelpantic]

This 3 lines wil be written in every callback, so I would factor them out into another static method.
On a second thought, we might need to change this a it, as we probably don't always have the clear assignement one sbp messsage => corresponds to one ros::publisher.

hmm maybe the context pointer here could be the PiksiMulti Instance itself, and then we have some ENUM Map to get the publishers? But I think there could even be a cleaner solution, we can discuss today ;-)

[michaelpantic]

Also we can have a discussion on how to make it nicely multithreaded and stuff. But can also happen at a later stage.