Code cleanup

Warning: these changes aren't fully tested, I am still waiting for my pyra ;)

Hello,

as discussed on discord, this MR contains various code cleanups.

Changes:

• make code style mostly PEP8-conform (damn tabs)
• restructure to the standard python package layout, allowing the use of package-specific features, reducing import ambiguity and allowing for a better UI/logic separation
• separate the UI from the rest of the functionality
• change various functions for 'better' alternatives (sys.exit() instead of os.exit(), subprocess.check_call() instead pf os.system(), ...)
• change CLI to use subcommands rather than flags
• replace some of the used dictionaries with new classes, making it clearer what which functions expect/return
• minor renames
• add documentation
• various minor bugfixes

Beware that I am not yet sure how the DBP creation process must be changed to support the new package layout.

Weirdly, I've just noticed that gitlab shows some files as deleted, despite git diff affirming that they were only moved. I've marked this MR as a draft for now while I try to figure out what's going on.