NEMA17 motor mount for 2020 - parametric

I made two small modifications to the original. Now, the mount middle bar really stays at the middle regardless of the size of the motor or the beam. Also, I made circles more "round" by adding the variable $fn = 64 to the open scad code. Still possible to improve by optimizing the positioning of the holes for fixing screws, but for now I will leave as it is. Thanks to ccox for the original design.