AP_InertialSensor: allow gyro calibration to take up to 30 seconds

this gives time for the user time to get their battery hatches on and
hold the model steady. 5 seconds was just far too short
AP_NavEKF: add another health check in the EKF

if SV, SP and SH are all off then the most likely cause is divergence
of the EKF. This was done based on a flight log with bad gyro cal
Replay: cope with new airspeed log msg
AP_NavEKF: added getStaticMode() function
DataFlash: added staticmode to EKF4 message

useful when diagnosing logs
AP_Math: fixed build warning
RC_Channel: added set_radio_trimmed()

take into account trim value in set_radio()
Plane: use set_radio_trimmed() for flaperons

See discussion in issue #1425
GCS_Mavlink: New message def: MAV_CMD_NAV_CONTINUE_AND_CHANGE_ALT
GCS_MAVLink: include units in CONTINUE_AND_CHANGE_ALT
GCS_MAVLink: code generation from XML for MAV_CMD_CONTINUE_AND_CHANGE_ALT.
AP_Mission: support for MAV_CMD_CONTINUE_AND_CHANGE_ALT
Plane: Implementation of MAV_CMD_CONTINUE_AND_CHANGE_ALT.
Plane: Keep throwing WP in front plane for CONTINUE_AND_CHANGE_ALT
Plane: fixed CONTINUE_AND_CHANGE_ALT command

current_loc should be used, not prev_WP_loc
AP_Mission: Support MAV_CMD_DO_FENCE_ENABLE as a mission item.
Plane: Support MAV_CMD_DO_FENCE_ENABLE as a mission item.
Plane: fixed missing break statement
AP_Math: fixed example build
Plane: prepare for 3.2.0 release