Z rod cap for Anet A6 MKII

I've made a redesign on Z rod cap for Anet A6 by jadames trying to reduce the friction against the frame and making the insert into threaded rod a little bit easier.
The main changes are:
- Changed the profile from straight cylinder to 3 toroid trying to minimize contact surface with the frame but allowing the cap to be centered.
- The rod insert hole now has cone shape to be able to insert it a little bit easier.

Maybe this improvements are like trying to reinvent the wheel, but maybe som