Improve the code style
Improve the code style

Adds docstrings, fixes some PEP8 problems, improves readability,
uses sets by default without a need to typecast.

jan-cerny/openscap,redhatrises/openscap,jan-cerny/openscap,OpenSCAP/openscap,OpenSCAP/openscap,jan-cerny/openscap,redhatrises/openscap,jan-cerny/openscap,OpenSCAP/openscap,OpenSCAP/openscap,OpenSCAP/openscap,OpenSCAP/openscap,mpreisler/openscap,redhatrises/openscap,mpreisler/openscap,mpreisler/openscap,mpreisler/openscap,jan-cerny/openscap,redhatrises/openscap,mpreisler/openscap,jan-cerny/openscap,redhatrises/openscap,mpreisler/openscap,redhatrises/openscap