sixtracklib/tests: add align and cavity beam-elements tracking unit-tests; cosmetic changes
sixtracklib/tests: add align and cavity beam-elements tracking unit-tests; cosmetic changes

- Add unit-test coverage for the NS(Align) and the NS(Cavity) beam-elements on the CPU
- Fix occurances where comments referred to the wrong type of beam-element due to copy&paste
  errors

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