Paths with constant distance

The example shows how to create paths with constant distance. This is achieved
with the aid of `deformer.parallel`, which has already been used in the above
example. ...

The first part of the example code is only for creating the middle line of the
knot. In order to obtain a nicely curved rope, we smooth the such defined curve
a bit.

The individual parts of the knot must be stroked in a very specific way in
order to achieve the correct overlapping of the knot. To this end, we split the
base curve into five pieces.

The key element of the example is the use of `deformer.parallel` which creates
the curves with constant distance from the base curve. We apply the deformer
four times, thus creating the outline path of two individual ropes, which
parallel follow the shape of the knot.
