These methods are essential to identify where to split the line-of-sight
in the Duvenhage algorithm, when going from one level in the min/max
kd-tree to the next level.

Elevation is going to be interpolated at tiles entry and exit points,
which are computed from geodetic conversions. The result points may be
very slightly out of tiles, at numerical accuracy level. These points
should nevertheless be allowed to be interpolated using the closest
pixel data, so a tolerance was needed.

The SimpleTile class uses a fixed tolerance, arbitrarily set to 1/8
pixel.

Tiles are expected to have no inter-tile gap, i.e. the boundary row/columns are repeated in neighboring tiles.

This allows them to chop off line-of-sight at DEM cells boundaries.

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.