AP_NavEKF: Inertial Navigation Code - 24 State EKF

initial version converted from matlab
AP_NavEKF: Updated GPS input processing
AP_NavEKF: Updated measurement fusion control
AP_NavEKF: Corrected height update timeout error
AP_NavEKF: Added constructor for measurements
AP_NavEKF: Updated measurement references
AP_NavEKF: fixed build for APM environment
Plane: added NavEKF object
AP_NavEKF: Explicitly defined numerical constants as single precision using f suffix
AP_NavEKF: Updated comments in header file
AP_NavEKF: initial test sketch for NavEKF
AP_NavEKF: added dataflash log reader

this gives log playback into HIL
AP_NavEKF: added euler angle functions
AP_NavEKF: try running EKF on log data
AP_NavEKF: fixed compass declination, baro cal and time shift

now runs much faster
AP_NavEKF: added plot.dat output
AP_NavEKF: debug updates
AP_NavEKF: added convenient plotting scripts
AP_NavEKF: added pause to gnuplot scripts
AP_NavEKF: added write_flot valgrind functions