This PR adds support for several features in the `natural` key, including `saddle`, `ridge`, `arete`, and `cliff`. These features all currently render in openstreetmap-carto, so adding these features would advance OpenMapTiles towards its goal of parity with the standard renderer, as indicated in https://github.com/openstreetmap/operations/issues/565#issuecomment-907303115.
This PR also adds the features requested in #274, with the exception of valleys, which I've left out of this PR because there are different complexities to mapping valleys that should be addressed as a separate (future) PR.
### Examples of mountain features in paper maps
These features are regularly found in American-style maps and thus is of interest to the openstreetmap-americana map style. Below are examples of these mountain-related features found in general purpose maps:
<img src="https://user-images.githubusercontent.com/3254090/131270340-af80c7bf-9416-40a3-9b64-56418abb2aef.png" width=400/>
<img src="https://user-images.githubusercontent.com/3254090/131270375-b9eb2095-7708-443d-b7bc-22bf8adab721.png" width=400/>
<img src="https://user-images.githubusercontent.com/3254090/131270423-64447ad3-d90a-4615-82e1-91175a8f8a6b.png" width=400/>
<img src="https://user-images.githubusercontent.com/3254090/131270506-2248db9f-ded5-443f-82ed-b0e8d4d12a70.png" width=400/>
### Approach
This PR extends the existing `mountain_peak` layer by adding other mountain features. We may want to consider renaming this layer in version 4.0 to be more inclusive of other mountain features including the potential future addition of valleys. However, the features added in this PR are associated with mountains and so their inclusion in this layer is the most appropriate location.
A new `osm_mountain_linestring` mapping maps the new linear mountain features, with similar ranking logic as is used for the existing `peak`/`volcano` features. Additionally, `natural=saddle` is added to the `osm_peak_point` mapping and ranked using the formula for peaks. Since saddles will have lower elevations than peaks, important saddles will be preempted by important peaks.
The new linestring features are rendered only at zoom 13-14, in order to match the zoom at which they appear in openstreetmap-carto. However, it may be appropriate in a future PR to extend the rendering of these features as generalized linestrings at lower zooms.
### Test Renderings
Below is a test rendering showing aretes and peaks in Austria, just north of the Swiss border, followed by a screen shot of the [same location](https://www.openstreetmap.org/#map=14/46.8682/10.0863) in openstreetmap-carto:
**OpenMapTiles, zoom 14**
<img src="https://user-images.githubusercontent.com/3254090/131271258-5cd90bdb-cac2-41d8-887f-b4bf6be83673.png" width=400/>
**openstreetmap-carto, zoom r14/v13:**
<img src="https://user-images.githubusercontent.com/3254090/131271332-32d5bcfc-41c6-4625-829c-df063b7af523.png" width=400/>
quicker and cleaner diagram image generation.
Remove etl-graph and mapping-graph targets - redundant
Also, the obsolete "fields" is still in Imposm's code and both names are accepted,
but "fields" is not documented anywhere (PR submitted), and could be removed at any moment.
Our docs were not supporting it until this PR, so renaming it at the same time.
Several images have been updated due to a more inclusive mapping scan
Requires https://github.com/openmaptiles/openmaptiles-tools/pull/147 (merged)
Brian Sperlongano
