Function initial_bearing_deg 

pub fn initial_bearing_deg(from: Position, to: Position) -> f64

The bearing to leave from on to reach to by the great circle, in degrees true.

Only the initial bearing: on any leg that is not along a meridian or the equator, holding this bearing does not arrive at to. See final_bearing_deg for the other end, and crate::rhumb for the constant-bearing alternative.