Parameterized printable spring

It is a proof of concept of a kind of printable spring. Initially I designed it for using in my Prusa Mendel, because the z axis springs were very difficult to find for me. Then I generalized the idea and parameterized the spring.

These springs conceptually consist of a strip of plastic that is bended. The outer cross section of the spring can be square, hexagonal or octogonal.

The spring parameters are:
* fid (fold inner diameter)
* folds: Number of folds
* th: strip thickness
* Dril