AP_L1_Control: prevent another NaN in L1
autotest: raise timeout to 7000 seconds
Copter: report baro health to GCS
Plane: report baro health to GCS
Baro: make get_altitude_difference const
Baro: add altitude sanity check

healthy flag made protected
healthy accessor fn added which also check latest calculated altitude
was ok
Baro_BMP085: use healthy flag
Baro_HIL: use healthy flag
Baro_MS5611: use healthy flag
Baro_VRBrain: use healthy flag
Baro_PX4: use healthy flag
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 warning
BaroGlitch: check baro health after alt calculation

Also correct type of distance_cm internal variable