Identified a new failure mode for polygons.
Improved accuracy of internal computation.

The improvement is obtained by using high accuracy linear combinations
and by preserving cos/sin as much as possible. This last part is
important in the frequent case of lines along y axis, i.e. when angle is
π/2. We want the cosine to remain 0 and not get about 10^-17.
Reorganized code, avoiding too many internal classes.
Added splitters in Boundary attributes.
set up a weak link between a line and its reverse to avoid duplications.
Removed development traces.
Fixed a problem with too thin polygons considered to have infinite size.
Deprecated AVLTree and OrderedTuple.

These two classes were dirty hacks used only for the extraction of
boundary from 2D Euclidean BSP trees. They were not really efficient and
produces some problems as topology was not preserved.

A completely new implementation of boundary extraction has been set up
as of 3.4, that preserves more topological information and therefore
should be more robust. This new implementation does not use these
classes anymore.

As AVLTree is not really a math concept, this class is not considered to
belong to [math] scope. OrderedTuple is more a grey zone, but it is
really something inefficient and no need for it is foreseen.