Copter: Log NTUN while in LAND mode with GPS
Frame_Parameters: 3DR_Iris+ changed mode switch configuration
AP_NavEKF : Clean up time stamps

Time stamps are now explicitly initialised to the current IMU time to avoid unwanted activation of timeout logic on filter start and various calls to the hal.scheduler->millis() object have been consolidated.
AP_NavEKF : Reduce ripple in estimates that can cause copter motor 'pulsing'

This patch reduces the level of 5Hz and 10Hz 'pulsing' heard in motors due to GPS and altimeter fusion which cause a small 5Hz and 10Hz ripple on the output under some conditions. Attitude, velocity and position state corrections from GPS, altimeter and magnetometer measurements are applied incrementally in the interval from receiving the measurement to the predicted time of receipt of the next measurement. Averaging of attitude state corrections is not performed during periods of rapid rotation.
AP_NavEKF : Reduce sensitivity on filter divergence check

Flying aerobatics with Trad Heli has shown that the divergence check can be false triggered when large magnetometer errors and GPS dropouts are present.
This can also happen with multi rotors if large yaw rates are present.
This was an unintended consequence of the ekfsmoothing patch which improved filter stability during high rate manoeuvres, but made the divergence test more sensitive.
EPM: add params for pwm levels add re-grab feature

Also include some minor commenting changes
Copter: remove EPM neutral from ch7/ch8 switch

The EPM library returns the pwm output sent to the EPM to the neutral
position automatically meaning an explicit ch7/ch8 setting is not
required