Generate+display thumbnails/overviews for coverages

[IvanSanchez]

Briefly discussed in person with with @tomkralidis, during the OGC/OSGeo/ASF codesprint in Évora.

Is your feature request related to a problem? Please describe. The goal is to improve the user experience when browsing collections containing coverages. As a user, I would like to see a preview of what the coverage contains instead of just its bounding box.

Describe the solution you'd like The proposal is to add a field to the configuration of coverages: either thumbnail or preview or overview, like e.g.:

        providers:
            - type: coverage
              name: rasterio
              data: data/stuff.tif
              thumbnail: auto

For (very) large datasets, generating a thumbnail/overview might be troublesome, and data providers might already have overview images, so it might be also possible to think of something like:

              thumbnail: data/stuff-preview.png

Additionally to the thumbnail/overview generation, it should be displayed in the collection/coverage webpages when browsing - that means a bit of Leaflet work.

Describe alternatives you've considered Assuming that the coverage is a GeoTIFF, fetching the overview (or the 0-th image in the file) is not a viable solution because of the lack of HTTP range requests.

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