Improvements for Geeetech G2S

Parts to improve the print accuracy and quality of Geeetech G2S delta printers. For detailed description see the following web site:
[Bechele.de - english] (https://bechele.de/?page_id=496)
Updated 03.Feb.2018:
Because I noticed that I could not tigthen all three screws for the lower bearing holder because of mechanical inacurracy that caused hard going bearings, I modified the lower bearing mounting using thread bolts. The thread bolts allow to adjust the parallelity of the upper and lower bearing very exactly. This is achieved by adjusting the nuts carefully.