By convention, `requirements.txt` is used to describe concrete Python
environments, but in this project the file was used to describe package
dependencies. Thus, remove `requirements.txt` and declare dependencies
in the `setup.py` script directly.

Add Sphinx extension and limited Python 3 compatibility

See merge request !1

SVG files can now be used in LaTeX documents with the `embedded-image`
directive. SVGs are converted to PDFs with `rsvg-convert`.

- Added a recommendation for `pdf2svg`

Images are always embedded in the output PDF file, so embedded images
can be handled like normal images.

Before this commit, the fzj logo was not cleaned up.

- Suits corporate design requirements better
- More flexible: the document type can be configured by defining a
  `\documenttype` command