Replace helicity loops with individual threads

[roiser]

• Looked into (Mat), last comment was that this showed a 3 % improvement

• code example in Mat's github

[valassi]

Note: in the vectorization work #71 and #72 the helicity and event loops get reversed. So this would look very different than at the time of the hackathon. Keeping open, maybe still useful to investigate for very complex processes.