Simplify volume rendering example

It was a bit of effort to try out the original example, since you had to pass in an H5 file. This PR just creates a simple example volume at run time.

A couple of things I noticed while playing around with this:
* The volume doesn't get displayed if I don't pass the spacing parameter to `VolumeData`.
* You can drag a control point on the transfer function to the very edges of the colorbar, which make it very difficult to move later.