Global position plugin

[TSC21]

Here's the inital release of the global position plugin. Needs completion and testing!
There are some thing that must be added though (not sure how to do it):

• See how to receive time_boot_ms and how to convert it to ros::time();
• Probably we have to consider the NavSatStatus here too, but I'm not sure.

Please @vooon feel free to give an opinion and some tips on this.

The gps_conversions.h file also allow us to convert from local UTM to GPS coordinates, which can also be good on the APM case (considering that this one accepts more than just NMEA convention).

Fix #87.

[vooon]

I added Fix #87 to PR description. Maybe best to place gps header to include dir?

[vooon]

See how to receive time_boot_ms and how to convert it to ros::time();

I don't do it. Easier use ros::Time::now().

[vooon]

I think i can merge it now and do corrections.

[TSC21]

I don't do it. Easier use ros::Time::now().

Did you find out if PX4 can send GPS time?

[TSC21]

I'm going to do some corrections to complete what you merged so far already ;)

[TSC21]

Relative to the position_covariance_type: I think is going to depend if we send the covariance from the PX4 or not right? The covariance being used right now is just to fill the PoseWithCovarianceStamped message, but this covariance can be issued from the PX4 also.
So, for now, do you agree we set it as COVARIANCE_TYPE_UNKNOWN? This off course is to stay just 'till we have covariance published from PX4.

Can you add the Status part of the message please? I'm not really sure how to do it, even based on what you did in gps.cpp.

And that way we will have all fields of NavSatFix filled.

[vooon]

To calculate NavSat covariance i need EPH value, sended in GPS_RAW_INT.
I do it later, today repairing bixler (repair "duck face" after crash).

[TSC21]

OK. Good luck with the repairing then ;)
Can you please also add the NavSatStatus so to fill all the NavSatFix msg fields? Thanks!