Improved Ball and Socket Joint for PCB Workstation

This is an improved ball and socket joint for giufini's excellent [PCB Workstation with Articulated Arms](http://www.thingiverse.com/thing:801279).

The author used HIPS for his ball and socket joints but I only print with PLA and PETG. With both materials, I ran into a number of problems getting the articulated arms to work well. Here's a summary of the problems I had:

1. The socket ball wears quickly and the articulated arm becomes very loose
1. The ball easily pops out of the socket when the travel limit of the arm is reached
1. The arms don't bend with a very tight radius so moving the arm around frequently causes the balls to pop out

I tried both versions of the alternative ball and socket design with heat shrink tubing but the heat shrink quickly breaks down and gums up the socket. Plus, the bend radius is also reduced with the heat shrink.

So I took a stab at designing an improved ball and socket joint. The biggest requirement is that the ball remain the same si