Given two points in ECI reference frame, check wheather they are in line-of-sight, i.e, they are not obscured by Earth.
Note that the Earth is assumed as a sphere with 6,378,137.0 m radius (equatorial radius according to WGS-84).
x1: [x y z] ECI coordinates of the first reference point
x2: [x y z] ECI coordinates of the second reference point
inLOS: true if the line-of-sight is not obscured by Earth