Vector data clamping to ground

[vpicavet]

Right now vector data are displayed, but not clamped to the ground.
We will need to be able to clamp the data for 2D vector data visualization :

• Points
• Lines
• Polygons

@vmora already implemented a clamping algorithm previously, he may be able to help.

Clamping should be usable by all loaders for vector data, like WFS ( #115 ), GPX ( #114 #158 ), KML ( #24 ), GeoJSON, SHP and vector tiles.

[gchoqueux]

Some functions to add

• map.getCurrentAltitude(longitude, latitude)
  This function returns altitude of the current MNT displayed
• process.PlaceMeshOnMapSurface(node,map)
  This process place the node/object3d on 3d surface of map

[gchoqueux]

This features are in itowns