autotest: fixed multi-instance support
autotest: initial version of a antenna tracker simulator
autotest: added -T option to sim_arduplane.sh

starts an antenna tracker
AntennaTracker: fixes for antenna tracker

- use pressure to calculate vehicle altitude
- changed approach to slewing
- fixed mavlink mode
AntennaTracker: added a note on simulating an antenna tracker
AP_NavEKF : Update comments and remove un-used function declarations