Subset of Marco's changes

This mr is a subset of marcos changes in !10 (closed).

It contains the following portions:

• Smaller Bug/Stylefixes
• Move to BibLaTex over BibTex
• latexmk configuration
• TikzExternalize

It deliberately omits the following portions:

• minted: I do not deem that a good decisision from a portability/fragility point of view
• glossaries-extra: Never used it, cannot really vouche for it
• biblatex author configuration: I believe that in a general purpose template, "et al." is the better option.

I've split out the latter three changes and will submit them in a second MR that can be discussed/reworked on its own.

Thus this mr thus fully superseeds !3 (closed), !4 (closed), !5 (closed) and !7 (closed), and partially addresses !10 (closed).

Reviews welcome.