Function calculate_direction 

pub fn calculate_direction<T: Float>(
    lat: Angle<T>,
    lon: Angle<T>,
    azi: Angle<T>,
) -> Vector3<T>

Calculate the direction vector along a Great Circle from an initial position and an azimuth.

See: Panou and Korakitis equations: 30, 31, & 32a https://arxiv.org/abs/1811.03513

• lat - start point Latitude.
• lon - start point Longitude.
• azi - start point azimuth.

returns the direction vector at the point on the great circle.