The cases handled include linear cases (when Digital Elevation Model has
zero curvature) and constant cases (no curvature and line-of-sight
parallel to tile, both in or out-of-tile).

The case where the line-of-sight enters the Digital Elevation Model in
one tile on top and exit it in another tile on bottom is not tested yet.

The test does not succeed yet, an infinite loop is triggered.

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.