Allow changing color scale size

[Conengmo]

Closes #72. Allow changing the size of any color scale.

color_scale = StepColormap()
color_scale.width = 1000

I didn't make it a function parameter since I'm not sure how widely used this feature will be. But with this change at least the variable is no longer hard-coded but exposed and can be changed if wanted.

[Conengmo]

In Colormap.render() I didn't include self.height, because the height of 40 is hard-coded throughout that method.