Beta casted Mendel parts

This is a complete parametric version of pourable mendel parts.

All Mendel parts are openscad models now. Some of them where fortunately already made by Tim Schmidt (http://github.com/timschmidt/parameterized-mendel).
All others where completly recreated in openscad after the original stls. This allows to make stls with adapted hole sizes, so they have the correct size when printed on your 3d printer.
No drilling needed.

Some parts where changed to make them easier pourable, those mod