"Gaussian" CTF nodes.

A single color, plus opacity going from zero to a peak then back to zero.
It's basically a nice way to fake isosurfaces when used with the volume
renderer.

It's "Gaussian" for now, because the opacity is still linear. That
will be fixed soon.
Oops... Fix some code that was missed during refactoring.
Be more strict about nodes in GaussianComponent.from_function_nodes
Add a note about the gaussian radius resize behavior.
Merge pull request #29 from enthought/feature/gaussian-ctf-node

"Gaussian" CTF nodes