Latex cleanups

This is a (suprisingly) huge cleanup of the latex doc.
The main things are

 * restructured preamble:
   * system stuff is now in `common/system.tex`
   * stylistic stuff in `common/style.tex`
   * new commands and macros in `common/commands.tex`
 * removed unused packages from loading
 * replaced how-to graphic with cross-referenced linine-tikz picture
 * unified handling of epigraphs and citations
 * use the KOMA-script customization commands instead of external packages where possible
 * avoid deprecated (eg `\it`) or uncommon (eg `\input{"./foo.tex"}` usages