AP_Math: fixed example build
AP_NavEKF: don't build example
AP_Baro: fixed example build
AP_HAL: fixed example build
AP_Notify: fixed example build
Plane: enable Baro logging
AP_AHRS: give zero lat/lng and baro alt if no position yet in DCM
AP_AHRS: move using_EKF to .cpp to make it easier in gdb
AP_NavEKF: force healthy false when initialising

this prevents us feeding off our own values when booting

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
AP_Math: added quaternion.from_rotation_matrix()
AP_Math: added tests for quaternion to/from rotation matrix
AP_NavEKF: initialise quaternion from rotation matrix
Compass: split compass learning out to its own C++ file
DataFlash: periodically use lseek to avoid NuttX seek bug
AntennaTracker: change to Compass::learn_offsets
Plane: change to Compass::learn_offsets
Copter: change to Compass::learn_offsets
Rover: change to Compass::learn_offsets
AP_Compass: constrain compass offsets to +/- 2000

this is prompted by a user log showing NaN compass offsets, resulting
in a crash. The patch ensures we never end up with NaN offsets, and
also constrains the offsets to a reasonable limit
AC_AttControl: accel limit for roll, pitch yaw rates