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_GPS: fixed num_sensors on APM2