Replace the original attempt at providing step-53 by something real.

In particular, provide a step-53 that shows how to describe geometries
using charts and use a section of Earth under Eastern Africa as an
example. This has about the right level of complexity, and produces
awesome pictures.

This commit was generated from a number of smaller one:

More text.

Implement the Africa model for the WGS 84 geometry (actual topography still missing).

Provide the actual data set. Provide a way of reading it when not compressed.

Results and finish the program. Much still to be done.

More text.

Finish documenting everything.

Remove a picture we now no longer need.
Provide some cross-references to the new step-53.
Show some more results.
Downsample the data file by a factor of 3 in each direction. Adjust the program accordingly.
Add a changelog entry.
Clarify where the data comes from.