Example: Process simulated seismic volume.

This PR provides an example for DistArray, namely the processing of a synthetic seismic volume from an HDF5 file.

The file `create_volume.py`, which does not use DistArray, creates an HDF5 file, containing a 3D array, which is intended to resemble a seismic volume. I have tried to make the data look somewhat interesting.

The file `load_volume.py` is the substantive part of this example. Namely, it is a DistArray program for manipulating the data created by the first program.

This loads the dataset, takes three slices of the volume, and creates matplotlib plots of them, then calculates various per-trace statistics, and then applies two different filters (a 3-point average, and a 3-point maximum) to each trace. The 3-point average result is then sliced and plotted, and saved as both `.dnpy` files and as a new `.hdf5` file. It can also compare the results from DistArray operations to that of normal numpy operations on the full array.

The array size is indeed now controllable via command line, and I have updated the README to note that, and how to control it. I did not try and document all the valid command line arguments, the `--help` argument will describe everything (hopefully).