fix docs
make single_evalverschikcung and evalverschickung work
consolidate hold_button
improve tutor eval:
* generate with preface
* give proper PDF title
* clean up after generate
expose print tutor eval better
enforce minimum returned sheets policy for tutor result generation
also clean generated tex
fix #2
not sure why this is required, but it fixes TeX
only show abstensions if there were any in result.pdf