streamlining the code

[dmnk]

during the work on #81 I found the check method to be a bit on the long side and do a lot of different stuff in the main "check" function.
I'd refactor some parts for clarity, and provide a dedicated pull request for them - to keep the changes traceable.

main "clean-code" targets that should be achieved:

• reduce function length: 1 page per function max
• scope the responsibility (prechecks, running other functions / logic, manage results / outputs)