Hyperboloid pencil holder

A pencil holder that creates a hyperboloid surface.

Although inspired by Stuffperson's hyperboloid pen holder https://www.thingiverse.com/thing:1401060 this is a from-scratch re-implementation that adds some additional features I haven't seen on other similar things:
+ it has two concentric rows of pencils
+ it does the maths for you to prevent intersecting pencils
+ some tidy detailing:
+ the top of the peripheral wall is inclined at the same angle as the cups
+ the peripheral wall can be stepped down
+ the bottoms of the cups are rounded

In the customizer (or the OpenSCAD file) the key parameters are:
+ Pencil dimensions
+ Length sets how high the minimum width (waist) of the curve is - it will be halfway up the pencils if the pencils really are as long as you specify, but specify longer-than-true for a waist higher up the true pencils and vice-versa.
+ Diameter is diameter of the hole they go in, and also used to space the pencils so they don't intersect at t