swmr-demo

Initial commit of VFD SWMR demos: `credel` (cre)ates and (del)etes datasets and
groups.  `gaussians` generates a couple of 2-D Gaussian curves that move at a
constant speed inside of a rectangle and bounce off its sides.  `gaussians`
just displays.  `wgaussians` records the "image" of the 2-D Gaussians to an
extensible dataset.  `rgaussians` reads the dataset and uses `curses(3)` to
displays it as a "movie".