Plane: start working towards 3.1.1
Plane: use ground steering logic for final stage of auto landing

this should improve runway tracking
Plane: do landing flare if past landing point

this helps prevent us keeping the throttle on after we've landed if
the baro has drifted enough that we think we are not yet at the flare altitude
Plane: keep an estimate of the rangefinder altitude

only accept data when we have had 10 samples in a row in range at 50Hz
Plane: added RNGFND_LANDING option

this allows the use of a rangefinder for landing flare and landing
approach
HAL_SITL: fixed sonar correction for attitude
Plane: allow continued use of rangefinder data for 5s after loss of contact 

this allows short outages to be ridden out
AP_TECS: added TECS_LAND_TCONST

this allows control of the time constant for landing in TECS. A lower
time constant gives tighter altitude control on landing approach
Plane: improved landing glide slope

we project a point 500m past the landing point to prevent
discontinuites close to the landing point
Mission: start next nav cmd immediately after prev completes
Plane: lowpass the rangefinder correction, not height

this should produce less lag as the rangefinder correction should be
changing much less
HAL_Linux: removed retry loop on UART open

this was there for HAL_PX4 only, and makes no sense on Linux
AP_Vehicle: added land_pitch_cd for fixed wing
AP_TECS: make TECS aware of LAND_PITCH_CD

this makes the flare a bit smoother
AP_TECS: bring the flare height rate demand in slowly

bring it in over 0.5s to give less of a bounce
Plane: moved LAND_PITCH_CD into TECS
autotest: added CMAC_South

used to simulate takeoff to the south
AC_WPNav: resolve twitch when passing spline waypoints

The target positions target velocity was being reset to zero as we
passed through a spline waypoint.