Möbius strip

This is a solution for issue #249 and it is the result of contributions from [@deeplok](https://github.com/deeplook) and [@rpanai](https://github.com/rpanai).

from numpy import pi, cos, sin, linspace, meshgrid
import ipyvolume.pylab as p3

def möbius(draw=True, show=True, num=40, endpoint=True,
           uv=True, wireframe=False, texture=None):
    # http://paulbourke.net/geometry/toroidal
    u = linspace(0, 2 * pi, num=num, endpoint=endpoint)
    v = linspace(-0.4, 0.4, num=num, endpoint=endpoint)
    u, v = meshgrid(u, v)
    x = cos(u) + v * cos(u / 2) * cos(u)
    y = sin(u) + v * cos(u / 2) * sin(u)
    z = v * sin(u / 2)
    if draw:
        if texture:
            uv = True
        kwargs = dict(wrapx=not endpoint, wrapy=not endpoint,
                      wireframe=wireframe, texture=texture)
        if uv:
            kwargs.update(dict(u=u/(2*pi), v=v/(2*pi)))
        mesh = p3.plot_mesh(x, y, z, **kwargs)
        if show:
            p3.squarelim()
            p3.show()
        return mesh
    else:
        return x, y, z, u, v

mesh = möbius()

screenshot