Cesium2d viewer

[glughi]

What this PR does

Added Cesium in 2D mode (SCENE2D) to possible map viewers; WebMercator is set as map projection when using this mode. Cesium 2D is faster than Leaflet and the behavior of the two views (3d and 2d) in this way is homogeneous. Leaflet is not removed from the project and you can use one, the other or both.
Added the parameter mapViewers with which in config.json you can indicate which map viewers must be enabled and available in the MapSettings panel (the possible values are now "3d", "3dsmooth", "2d" and "cesium2d"). By default it keeps the known behavior.

Test me

Put "mapViewers":["3d", "cesium2d"], in your config.json and reload. In MapSettings panel you should see the new option.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[na9da]

@glughi - Thanks for this and other PRs you have created. We currently have a bit of a backlog but will hopefully be able to review yours soon :)