Local array plots of examples.

This PR adds plots of the local arrays for the example cases to the sphinx document. It also includes two new examples, an irregular block 2D example, and a 3D example, which has no plots. Note that the files here are automatically generated by code in distarray itself.

This PR also removes the examples for block-padded, as this is not actually implemented yet, so the plots were not very meaningful, and removes the example for block-cyclic, which is not working properly at the moment.

It also still has plots for the nondistributed `'n'` case. Should these be adjusted, or perhaps just removed?