Given an arc length (l) and segment length (delta) of the arc,
find where to put the center, returned as a position of the perpendicular
bisector of the segment.
A cubic Bezier curve can be defined by 4 points,
see http://en.wikipedia.org/wiki/Bezier_curve#Cubic_B.C3.A9zier_curves
For each of the curve end points, there is the last/first point of the
curve and a point which gives the direction and length of the tangent
vector on that side.
Reads: treeData.tree
Computes: treeData.nodes, treeData.degree, treeData.degrees
treeData.fathers, treeData.children, treeData.size,
treeData.values
Converts a tree to an array of nodes, in postorder.
A Bezier curve can be defined by four points,
see http://en.wikipedia.org/wiki/Bezier_curve#Cubic_B.C3.A9zier_curves
Here we give this four points and a integer to say in how many
line segments we want to cut the Bezier curve (if n is bigger
the computation takes longer but the precision is better).