Vorticity is now defined as the curl of the velocity. This adds a lot of noise sampling, possibly something that could be optimised although focus should be on parallelising the tracer update, that takes >85% of the running time. Looks good with properly calculated vorticity except that it is really weirdly twitchy. This is possibly due to the finite differences? Can we make this more accurate? Or should we just smooth the vorticity with a something lame like a moving average? Probably a good idea to address it from the vorton side though.