Refactor code into smaller pieces to avoid repetitive patterns

See https://gitlab.com/fabiangabel/research-topics-mat-tuhh/-/merge_requests/8

Several subroutines were created in order to refactor the large portions of repetitive code

• preproc_staff.sh handles substitution of staff keys like, e.g., fgabel
• preproc_wg.sh handles substitution of worging group keys like, e.g. aa
• preproc_topics.sh bundles the two former routines an loops through all .md files in order to create input files for pandoc

In order to maintain functionality of the basic preprocessor routines preproc_staff.sh and preproc_wg.sh a test_suite was implemented that checks the output against reference files.

the .gitlab-ci.yml was adapted accordingly. Stage names are now more meaningful for this particular project.