AP_AHRS : add method to report if EKF is waiting to start
AHRS: rename ekfNotStarted method to initialised

Also created default implementation in AP_AHRS class so AP_AHRS_DCM does
not need to implement it.
Copter: only report ahrs unhealthy after initialisation
Rover: only report ahrs unhealthy after initialisation
Plane: only report ahrs unhealthy after initialisation
TradHeli: remove overall throttle level from landing check
RangeFinder: TYPE param description to PX4-MaxbotixI2C

This hopefully reduces confusion for PX4/Pixhawk users with MaxBotix I2C
sonar
RangeFinder: TYPE param description to PX4-I2C
AP_NavEKF : Fix bug in reset of GPS glitch offset

The GPS glitch offset was being zeroed during position resets. This caused the filter to reject subsequent GPS measurements if the GPS error persisted long enough to invoke a timeout and a position reset.
Copter: use define for pre-arm compass offset check
AP_GPS: Adding support for the NEO7 GPS