Copter: Fixed GPS_ok() status checking

Fixed the GPS_ok() so that TRUE is returned in the cases of DGPS and Assisted GPS fixes, not just 3D fix.
SITL: support -P option

this allows changing a parameter on initial startup
autotest: replace sim_*.sh with sim_vehicle.sh

much nicer interface for developers
autotest: added locations.txt

startup locations for SITL
autotest: added AVC location
AP_Baro: expose get_altitude_difference() 

this is useful for pressure altitude calculations
AntennaTracker: use get_altitude_difference()
APM_OBC: added pressure altitude termination
autotest: added OBC frame type
Plane: updates for OBC API change
SITL: added SIM_BARO_DISABLE parameter

useful for testing baro failure
AP_NavEKF : Enable calc of wind velocity when not using airspeed sensing

This patch also cleans up the logic associated with use of the synthetic
sideslip measurement so that it can never be used for a non fly-forward
vehicle type
Merge branch 'master' of https://github.com/priseborough/ardupilot

* 'master' of https://github.com/priseborough/ardupilot:
  AP_NavEKF : Enable calc of wind velocity when not using airspeed sensing