[CSG] Add Empty Shape

[joeng03]

Description

Fixes #289

Note: yarn lint --fix was run so some of the CSG files were also linted accordingly.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

// Menger sponge, see
// https://en.wikipedia.org/wiki/Menger_sponge

import { union, subtract,intersect, translate, rotate, scale, render, 
         cube, blue,rounded_cube, empty_shape }
from 'csg';

const c = cube('#88ccff');

const d = scale(cube(blue), 1.5, 1.5, 1.5);

const empty = empty_shape();

function beside(s1, s2) {
    return union(scale(s1, 0.5, 1, 1),
                 translate(scale(s2, 0.5, 1, 1),
                           0.5, 0, 0));
}
function beside_frac(f, s1, s2) {
    return union(scale(s1, f, 1, 1),
                 translate(scale(s2, 1 - f, 1, 1),
                           f, 0, 0));
}
function before(s1, s2) {
    return union(scale(s1, 1, 0.5, 1),
                 translate(scale(s2, 1, 0.5, 1),
                           0, 0.5, 0));
}
function before_frac(f, s1, s2) {
    return union(scale(s1, 1, f, 1),
                 translate(scale(s2, 1, 1 - f, 1),
                           0, f, 0));
}

function below(s1, s2) {
    return union(scale(s1, 1, 1, 0.5),
                 translate(scale(s2, 1, 1, 0.5),
                           0, 0, 0.5));
}
function below_frac(f, s1, s2) {
    return union(scale(s1, 1, 1, f),
                 translate(scale(s2, 1, 1, 1 - f),
                           0, 0, f));
}

function menger(s, n) {
    if (n === 1) {
       return s;
    } else {
       const f = 1/3;
       const m = menger(s, n - 1);
       const outer_row = before_frac(f, m,
                               before(m, m));
       const inner_row = before_frac(f, m,
                               before(empty, m));
       const outer_layer = beside_frac(f, outer_row,
                                 beside(inner_row,
                                        outer_row));
       const inner_layer = beside_frac(f, inner_row,
                                 beside(empty,
                                        inner_row));

       return below_frac(f, outer_layer,
                         below(inner_layer, 
                               outer_layer));
    }
}

const menger_sponge = menger(c, 4);

render(menger_sponge);

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes
• [X] Any dependent changes have been merged and published in downstream modules