AC_WPNav: set loiter accel to 1/2 of speed

Bug fix from Leonard.
Also fixed some formatting.
Copter: add EKF params to full param list
Rover: add EKF params to full param list
Plane: reserve NavEKF slot in param list even when not used

Although this wasn't a problem immediatley, if someone had added a
parameter after k_param_NavEKF it's position in eeprom could have moved
as AP_AHRS_NAVEKF_AVAILABLE was enabled/disabled
InertialSensor_PX4: resolve compiler warning
Git: ignore Build.APMrover2 directory
AP_Math: make is_nan const for quaternion and add .zero() for vector2
AP_NavEKF: allow states to be accessed using names as well as a Vector22
AP_AHRS : change initialisation requirements for EKF and use dynamic method
AP_NavEKF : Improved stability of Z accel bias