Functions
interpolate returns the point X along the line segment AB whose distance from A
is the given fraction “t” of the distance AB. Does NOT require that “t” be
between 0 and 1. Note that all distances are measured on the surface of
the sphere, so this is more complicated than just computing (1-t)a + tb
and normalizing the result.
interpolate_at_distance returns the point X along the line segment AB whose
distance from A is the angle ax.