Feature: non cubic bounding box and axes

[maartenbreddels]

Example:

import numpy as np
import ipyvolume as ipv
x = np.arange(0, 100, 10).astype('float32')
xs, ys, zs = x, x**1.2, x**0.8

fig = ipv.figure()
s = ipv.scatter(xs, ys, zs)
ipv.set_box_aspect_data()
ipv.show()