[Proposal] Feature for souterrain/underground data visualization

[AnthonyGlt]

Context

Having a feature for souterrain/underground data visualization could add a new dimension in iTowns.

Proposal

Having the possibility to see through the ground to look at underground data such as sewer, fiber cable, subway line etc...
The globe/plan opacity will decrease and show this kind of data.

Implementation

Must hide the skirt #2280
Disable the atmosphere and change the background colour of the scene, or else it will be blinding
Decrease the globe/plan opacity

Example of the code to achieve the wanted result:

view.tileLayer.hideSkirt = true;
view.tileLayer.opacity = 0.2;             
const atmo = view.getLayerById('atmosphere');
atmo.visible = false;
view.scene.background = new itowns.THREE.Color(0x000000);