flexible roundbox lib for polyhedron

* English
(German version below.)

If someone want to construct a cuboid with rounded corners and edges, most people use the following construct:

minkowski()
{
cube([20,40,10], center=true);
scale([4,11,5])
sphere(r=1,$fn=32);
}

If you want just a rounded cuboid, this creates the desired result, but only that. If you want any further modification, you are restricted to what openSCAD gives you, i.e. `difference()`, `multmatrix()`, e.t.c..
Other interesting modifications like to swage one side or a trapezoid deformation are not possibloe after having passed the values to openSCAD.
A remedy is the use of the openSCAD module [`polyhedron()`](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#polyhedron), that enables you to construct any shape. Many people though don't use this possibility, because the parameters, which are still easy to understand, are not easy to calculate for everyone.

To ease the use of `polyhe