Support 3D with toplogy function

[dkastl]

It might be interesting to support geometries with 3rd dimension in the topology function. For shortest path function such as pgr_dijkstra the geometry does not matter anyway.

See: http://gis.stackexchange.com/questions/79996/how-to-build-topology-with-3d-linestring-in-pgrouting

[woodbri]

This is on my TODO list. I comes in two flavors,

1. where you have true 3D points
2. where we use 3D points to create separation of nodes based on z-levels at intersections

I responded to the gis.stackexchange with how to do this by cloning a couple of pgrouting functions.