Revert "Baro: sanity check get_altitude_difference"

This reverts commit 3650f10bbb9d222ef0f39143a7513d46da2ddbbc.
Baro: make get_altitude_difference const
Baro: add altitude sanity check

healthy flag made private
healthy accessor fn added which also check latest calculated altitude
was ok
Baro_BMP085: rename healthy to _healthy
Baro_HIL: rename healthy to _healthy
Baro_MS5611: rename healthy to _healthy
Baro_VRBrain: rename healthy to _healthy
Baro_PX4: rename healthy to _healthy
Baro: example sketch to use healthy() function
Arming: use baro healthy() function
BaroGlitch: use baro healthy()
Copter: use baro healthy()
Plane: use baro healthy()

Use baro last update time in place of healthy to determine whether HIL
sensor updates have started
Baro: init members to resolve compiler warnings
Baro_BMP085: init members to resolve compiler warnings
AC_WPNav: init members to resolve compiler warnings
AHRS_DCM: init members to resolve compiler warnings
GPSGlitch: init members to resolve compiler warnings
INS_HIL: init members to resolve compiler warnings
AC_MotorsHeli: init member to resolve compiler warning