Warning with plane SurfaceGrid

[tpaviot]

Making a plane surface geometry raises a warning:

surf_geo = SurfaceGeometry(z=[0] * 121, width=100, height=100, width_segments=10,
                           height_segments=10)

outputs in a jupyter notebook:

/lib/python3.6/site-packages/pythreejs/pythreejs.py:78: RuntimeWarning: invalid value encountered in true_divide
  uvs = ((positions - vmin) / vrange)[:, :2]

actually vrange is 0, thus leading to a division issue.

[vidartf]

I think the assumption being made is that a surface necessarily is defined by more than one point. I agree that the warning message is not very good at conveying this though. How would you expect this function to behave when passed a single point?

[vidartf]

I'm sorry, I read that wrong. I missed the * 121.

[vidartf]

Fix proposed in #160.