MPSM Customizeable Spindle Rounder

Inspired by @Enzi1984's design of [MPSM Spindle Rounder](https://www.thingiverse.com/thing:2036976) I've added to his goals (no supports, solid grip) mine - which as always are - make it customizable and divide & conquer (split to pieces to avoid reprinting the whole thing for each custimisation). Also I needed to add spacer to give some space to low-hanging RPi USB connectors. As result I've recreated the thing in OpenSCAD and added some parameters to make it customizable enough.

The spacer part is supposed catch off the spacer wings on original spindle and it has joint (central trench + pin) to keep the main rounder body part in place.