There was an issue since transformation.rst which uses this citation is
alphabetically after references.rst.
The documented workaround in https://github.com/mcmtroffaes/sphinxcontrib-bibtex/issues/8
is to rename references.rst as zrefereces.rst, so it is alphabetically last.

Modifies label to use BibTeX key name, and format names with lastfirst

Removed content sections from a bunch of pages since they clutter up the
PDF output and the same info is available in the sidebar of the webpage.

A few sections has been turned of for the PDF output, most notable the
front page of the webpage. It doesn't really fit in the PDF.

* replace SVG figure by PNG version,
* rewrite maths using :math:,
* put references in the main reference section,
* reformat references for consistency,
* put references in alphabetical order,
* use USGS URL for Snyder (1987).