Merge #322

322: Geosearch r=ManyTheFish a=irevoire

This PR introduces [basic geo-search functionalities](https://github.com/meilisearch/specifications/pull/59), it makes the engine able to index, filter and, sort by geo-point. We decided to use [the rstar library](https://docs.rs/rstar) and to save the points in [an RTree](https://docs.rs/rstar/0.9.1/rstar/struct.RTree.html) that we de/serialize in the index database [by using serde](https://serde.rs/) with [bincode](https://docs.rs/bincode). This is not an efficient way to query this tree as it will consume a lot of CPU and memory when a search is made, but at least it is an easy first way to do so.

### What we will have to do on the indexing part:
 - [x] Index the `_geo` fields from the documents.
   - [x] Create a new module with an extractor in the `extract` module that takes the `obkv_documents` and retrieves the latitude and longitude coordinates, outputting them in a `grenad::Reader` for further process.
   - [x] Call the extractor in the `extract::extract_documents_data` function and send the result to the `TypedChunk` module.
   - [x] Get the `grenad::Reader` in the `typed_chunk::write_typed_chunk_into_index` function and store all the points in the `rtree`
- [x] Delete the documents from the `RTree` when deleting documents from the database. All this can be done in the `delete_documents.rs` file by getting the data structure and removing the points from it, inserting it back after the modification.
- [x] Clearing the `RTree` entirely when we clear the documents from the database, everything happens in the `clear_documents.rs` file.
- [x] save a Roaring bitmap of all documents containing the `_geo` field

### What we will have to do on the query part:
- [x] Filter the documents at a certain distance around a point, this is done by [collecting the documents from the searched point](https://docs.rs/rstar/0.9.1/rstar/struct.RTree.html#method.nearest_neighbor_iter) while they are in range.
  - [x] We must introduce new `geoLowerThan` and `geoGreaterThan` variants to the `Operator` filter enum.
  - [x] Implement the `negative` method on both variants where the `geoGreaterThan` variant is implemented by executing the `geoLowerThan` and removing the results found from the whole list of geo faceted documents.
  - [x] Add the `_geoRadius` function in the pest parser.
- [x] Introduce a `_geo` ascending ranking function that takes a point in parameter, ~~this function must keep the iterator on the `RTree` and make it peekable~~ This was not possible for now, we had to collect the whole iterator. Only the documents that are part of the candidates must be sent too!
  - [x] This ascending ranking rule will only be active if the search is set up with the `_geoPoint` parameter that indicates the center point of the ascending ranking rule.

-----------

- On Meilisearch part: We must introduce a new concept, returning the documents with a new `_geoDistance` field when it passed by the `_geo` ranking rule, this has never been done before. We could maybe just do it afterward when the documents have been retrieved from the database, computing the distance from the `_geoPoint` and all of the documents to be returned.

Co-authored-by: Irevoire <tamo@meilisearch.com>
Co-authored-by: cvermand <33010418+bidoubiwa@users.noreply.github.com>
Co-authored-by: Tamo <tamo@meilisearch.com>

milli/Cargo.toml

@@ -16,6 +16,7 @@ flate2 = "1.0.20"
 fst = "0.4.5"
 fxhash = "0.2.1"
 grenad = { version = "0.3.1", default-features = false, features = ["tempfile"] }
+geoutils = "0.4.1"
 heed = { git = "https://github.com/Kerollmops/heed", tag = "v0.12.1", default-features = false, features = ["lmdb", "sync-read-txn"] }
 human_format = "1.0.3"
 levenshtein_automata = { version = "0.2.0", features = ["fst_automaton"] }
@@ -27,6 +28,7 @@ once_cell = "1.5.2"
 ordered-float = "2.1.1"
 rayon = "1.5.0"
 roaring = "0.6.6"
+rstar = { version = "0.9.1", features = ["serde"] }
 serde = { version = "1.0.123", features = ["derive"] }
 serde_json = { version = "1.0.62", features = ["preserve_order"] }
 slice-group-by = "0.2.6"