Clarify that current cross-track distance uses spherical (Haversine) earth model, add Ellipsoidal (Vincenty) version

[urschrei]

https://docs.rs/geo/latest/geo/algorithm/cross_track_distance/trait.CrossTrackDistance.html

One approach would be:

1. Deprecate cross_track_distance in favour of cross_track_distance_haversine
2. Add cross_track_distance_geodesic (I think we have all the necessary pieces between geo and geographiclib_rs)
3. Note the advantages / caveats related to WGS-84 (specifically: most WGS-84 coordinates won't be accurate to less than 1 m anyway)

[frewsxcv]

A PR was opened for this here: #1140

[frewsxcv]

(Didn't mean to close)