Local array distribution plots, plus even more examples.

This PR provides some more example plots for the protocol documentation, as included in #255, which this encompasses. This also includes bugfixes from #270, which should be merged first.

The example plot generator now produces a local array plot, showing how the arrays are distributed across the processes, and the generated sphinx documentation is laid out in a table, with the same layout as the local array plots. The plotting should handle any reasonable number of engines. The example generator expects at least 8 engines to provide an environment for the 3D example, which is without a plot.

This now writes the plots into a subfolder of the examples directory, I added a README file so that git will create the subfolder. (This arrangement is less cluttered and matches the protocol documentation better.)

This PR is basically in parallel with https://github.com/enthought/distributed-array-protocol/pull/91, which provides exactly the output of the example script.