Altitude management for Line in GeoJSON

[flavignotte]

Altitude is not taken into account for line when read from GeoJSON.
All lines are "clamped" into to terrain.
The user should be able to choose if we want to take the altitude read from GeoJSON or clamp the line on the terrain.

Sub-issues :

• Add onTerrain parameter for FeatureStyle : Add onTerrain parameter for FeatureStyle #2
• Tesselate lines to follow the earth curvature (Great Circle) : Tesselate lines to follow the earth curvature (Great Circle) #4