This is a first step towards duvenhage's algorithm.

This is work in progress, the min/max computation seems wrong for now.

Specialized tiles should directly extend SimpleTile.

For now, only the (incomplete) API and classes hierarchy has been set
up. The tile is still a simple tile and does not yet creates the min/max
kd-tree. The algorithm doesn't do anything.

The factory is to be implemented by the DEM intersection algorithm, at
very low level (typically tiles based on min/max kd-tree for Duvenhage
algorithm).

The tile updater on the other hand is the responsibility of the mission
specific interface.

This ensures simple search for tiles given a ground point.