Store the geopoint in three dimensions

[irevoire]

Related to this issue: meilisearch/meilisearch#1872

Fix the whole computation of distance for any “geo” operations (sort or filter). Now when you sort points they are returned to you in the right order.
And when you filter on a specific radius you only get points included in the radius.

This PR changes the way we store the geo points in the RTree.
Instead of considering the latitude and longitude as orthogonal coordinates, we convert them to real orthogonal coordinates projected on a sphere with a radius of 1.
This is the conversion formulae.

Which, in rust, translate to this function:

pub fn lat_lng_to_xyz(coord: &[f64; 2]) -> [f64; 3] {
    let [lat, lng] = coord.map(|f| f.to_radians());
    let x = lat.cos() * lng.cos();
    let y = lat.cos() * lng.sin();
    let z = lat.sin();

    [x, y, z]
}

Storing the points on a sphere is easier / faster to compute than storing the point on an approximation of the real earth shape.
But when we need to compute the distance between two points we still need to use the haversine distance which works with latitude and longitude.
So, to do the fewest search-time computation possible I'm now associating every point with its DocId and its lat/lng.

[curquiza]

⚠️ Should be merged once MeiliSearch v0.25.0 is released :) This fix is for v0.26.0

My bad there are already changed unrelated to v0.25.0 on main, so you can merge it!

[Kerollmops]

Hope the RTree works better with 3D points, will merge this in v0.26.

[Kerollmops]

bors merge

[bors]

Build succeeded:

• Run Rustfmt
• Tests on macos-latest with stable
• Tests on ubuntu-18.04 with stable
• Tests on windows-latest with stable