python/sixtracklib: updates beam_elements.py
python/sixtracklib: updates beam_elements.py

- updates the names and API of the space-charge API
- performs cosmetic changes
- applies linter to for pep8 conformity

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib