Round cubes

# Cubes with rounded corners for openSCAD

## Features

* Drop-in replacement for `cube()` function
* Design/prototype with cube() for fast preview and just replace it where needed.
* Optionally, specify a rounding radius for each cube. Default is 5% of
smallest dimension
* Skip rounding of top and bottom faces with `flat_tops` module
* `cube(x)` works with both `x` as a number and a vector
* Use `$fn=4` for chamfered edges - works great with `flat_tops`, but will cause some shrinking in `z` for `round_cube`.
* `round_square` is now an option as a 2D shape

## Usage

* Copy the `rounded_cubes.scad` file, and add `use `
before calling any of the modules.

## Example

```openscad
use

cube([10, 10, 10], $fn=20);
translate([15, 0, 0])
flat_tops([10, 10, 10], $fn=20);
translate([30, 0, 0])
round_cube([10, 10, 10], radius=1, $fn=20);
// For the twist to look pretty.
$fa = 1;
$fs = 0.4;
translate([45 + 5, 5, 0])
linear_extrude(height=10, scale=0.8, twi