Publish vessel pose via MAVLink

[clydemcqueen]

Publish the vessel pose (GLOBAL_POSITION_INT, ATTITUDE) via MAVLink, using a different system id (e.g., 2).

I believe that QGC will display and log it.

[clydemcqueen]

See also: https://github.com/zhrandell/Seattle_Aquarium_CCR_development/blob/main/1-pagers/GNSS_compass_logging.md

[clydemcqueen]

I did some experiments publishing GLOBAL_POSITION_INT with sysid=2.

• QGC logged these messages in the tlog file (good)
• QGC showed the vessel pose in the display (good)
• QGC turned on "multi-drone" mode, but the vessel-as-drone did not initialize correctly. I think this is because system 2 did not respond to the "get parameters" message (bad)
• QGC wrote a "vehicle2.csv" file, but the lat/lon were 0's (bad)

I'm a bit worried about making QGC mad... this might cause grief in the field.

I'm pausing this in favor of writing csv logs instead -- this will help us understand the data and make some decisions.

Related to zhrandell/Seattle_Aquarium_CCR_development#12