"Gaussian" CTF nodes

This is done enough to make it available to other projects. I'm still planning on making this actually evaluate a Gaussian for the opacity values.
This is more permissive that the corresponding methods on `ColorComponent` and `OpacityComponent`, since you could actually pass in any number of nodes as long as there was at least one of each required node. Do you want to also check the length of `nodes` here to match the other `from_function_nodes` methods?