Plane: added STAB_PITCH_DN_CD parameter

this adds some down trim when at throttle levels below the trim
throttle in FBWA mode.

defaults to 200 centi-degrees. I may adjust based on flight tests
AntennaTracker: do an initial pressure calibration on startup
AntennaTracker: added YAW_TRIM and PITCH_TRIM
autotest: fixed tracker sim for non-onoff mode
Copter: fix baro glitch resolved message in dataflash

Caught by Julien Dubois, thanks!
Copter: rate IMAX increase to 1000
InertialNav: do not reset velocity after glitch clears
Copter: increase ekf check threshold for inav

This increases the accel correction from 60cm/s to 80cm/s before the ekf
check will trigger a land
Copter: bug fix to land-detector

vehicle should become un-landed only when throttle out is above the
get-non-throttle-takeoff value
AC_PosControl: add set_alt_target_to_current_alt method
Copter: reset target alt to current alt when landed
Plane: more fixes for glide slope calculations
Plane: more accurate altitude tracking close to waypoints

don't use a glide slope if the altitude change is less than 15 meters
Plane: added GLIDE_SLOPE_MIN parameter

this is used to prevent slow glide slopes being used for small
altitude changes in missions. This allows more accurate tracking of
altitude with terrain changes
Plane: change threshold for no glide slope to 20m

40m is quite high for most planes
Plane: remved int cast

not portable
Plane: use bitfields in auto_state
Plane: prevent cross-track on some WP types

when finishing a takeoff or when a user forces a WP change don't do
any cross-tracking. This avoids a problem where the plane tries to
line up with a track completely unrelated to the track currently being
flown
AC_Motors: make get_throttle_out const
AC_PosControl: smooth take-off with accel PID's I term

This avoids an instantaneous jump in throttle during take-off by loading
the accel PID's I term with the expected change in throttle level