Switch to path and topologies geometries to 2D

Having 3D as primary geometries was a big design mistake.

Indeed: 

* Linear referencing happens in 2D
* Snapping happens in 2D
* QGis does not support edition of 3D geometries (see #688)
* Draping geometries using existing vertices only was wrong and imprecise
* Altimetric profiles required re-draping each time
* Most geoDjango stuff does not support well 3D
* For edition widgets, geometries had to be transformed from 2D to 3D and vice-versa

In this pull-request, we : 

* Add an additional field for 3D
* Store high-resolution draped 3D geometry
* Use 2D geometries for edition etc.
* Use 3D geometry to compute altimetric profile