Copter: enable GCS failsafe to RTL by default

This only triggers if the user has been using the GCS's RC override
SITL: prevent wind effects at negative altitudes

this prevents crashes on takeoff with barometer noise
AntennaTracker: fixed manual control, and added baro calibration

this allows baro offsets to be compensated for before takeoff
AntennaTracker: added SERIAL_CONTROL support

allows for radio update and config over USB
AntennaTracker: don't use EEPROM home if a startup home is set in parameters
AntennaTracker: added SCAN mode

this is used to test calibrate the compass, and to test tracking PIDs
AP_Motors: add stability patch test to example sketch
AP_AHRS: fixed wind reporting with EKF and no airspeed sensor

the EKF does not estimate wind without an airspeed sensor
Plane: fixed a bug in PWM based throttle failsafe

many thanks to Sam Tabor for finding this bug!
Plane: prepare for release of 3.0.1
AP_GPS: fixed a build warning

Debug() should not be exposed in public headers
DataFlash: added separate logging of each GPS
Plane: ensure we log both GPS at the time we receive a msg
Copter: ensure we log both GPS at the time we receive a message
Rover: ensure we log both GPS at the time we receive a message
AP_TECS: Parameter TECS_LAND_SPDWGT allows custom landing speed weight.
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