Add support for 3d displays

[gjmooney]

#51

• Support adding terrain objects
• Add UI element to add terrain
• Add Raster DEM sources to source panel context menu
• Add hillshade layers to layer panel context menu
• Add palette command to remove terrain

[github-actions]

👈 Launch a Binder on branch gjmooney/jupytergis/3d-terrain

[martinRenou]

Neat!!

In follow-up PRs:

• Similar to the approach we take to save the map center and zoom level in the file, we could save the pitch and rotation of the map. That would make sense now that we have a way to load 3D data
• It would be great to have a color picker in the hillshade layer form (See base/src/formbuilder/objectform/vectorlayerform.ts for an example of this):
  

[martinRenou]

Thanks!