kossel magnetic ball stud carriage redux

loved the original design, changes for this version:
- extended the back so that I did not need to print a separate wheel plate for delrin wheels.
- added 6 hole option, for 3, or 4 wheel configuration.
- bored the holes so that you can use 30mm M3 screws with delrin wheels.
- also added a tensioner for use with 3 delrin wheels.