Online access arctic dem

[trey-stafford]

Description

Add an online-only Arctic DEM. Related to #797

This adds a new "Arctic DEM" layer to "Terrain models/Internet required". The layer is grayscale but does have a legend. Unfortunately the legend does not have a label indicating units.

This layer is configured to use the arcgismapserver provider, which seems to provide a similar experience as WMS. The arcgis service does provide a WMS interface, it provides the same legend & grayscale data view. In both cases the identify tool and value tool cannot be used to inspect data values. Is this worth adding?

Checklist

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• The PR description links to issues that it resolves with closes #{issue_number}
• Config lockfile updated (inv config.export > qgreenland/config/cfg-lock.json)
• Environment lockfile updated if needed (conda-lock)
• Version bumped if needed (bumpversion (major|minor|patch|prerelease|build))
• CHANGELOG.md updated (for user-facing changes)
• Documentation updated if needed
• New unit tests if needed

[MattF-NSIDC]

Looks great! Pleased with how easy the new online access provider was to add.