ParametricSpring  (doc)

There is a spring in CustomElements library named ParametricSpring ([link](https://github.com/BriefFiniteElementNet/BriefFiniteElement.Net/blob/master/Sources/CustomElements/Elements/ParametricSpring.cs)). this should be moved into main BFE code, and an example about it to be added to documentation (something like beam on elastic foundation etc.)