AP_Baro : Indicated to true airspeed scaler and improved altitude accuracy

The baro altitude calculation has been adjusted to use the exact equations for the international standard atmosphere tropopause ( 0 - 11km AMSL). The old calculation would have the plane flying 300m low at 5000m with the error increasing exponentially with height.

Also a method for calculating the scaler from equivalent/indicated to true airspeed has been added. This can be used by speed/hight control and other navigation functions.