Added

• Python-based tests for the produced PDFs (1746e4e).
  This allows the user to automatically run a test suite against the produced PDFs,
  for example checking for page count, metadata and much more.
• Makefiles for the root and tests directories, while
  also swapping all CI routines over to use make (b4a9881).
  This allows for local as well as CI use using the same commands, and reduces coupling
  with the CI engine.
• Caching of files in CI, for much faster pipelines (while introducing some issues) (28aea76).
• Showcase and fixing of \abs macro for absolute values (4d0c6ff, bb11b72).

Changed

• Git metadata display in the colophon (7c59cbe).