tokens: WE and MJH acknowledge support from the UK's Science and Technology Facilities Council [grant numbers ST/M503514/1 and ST/M001008/1]. This work has made use of the University of Hertfordshire Science and Technology Research Institute high-performance computing facility. This work was supported by funding from Deutsche Forschungsgemeinschaft under DFG project number PR 569/10-1 in the context of the Priority Program 1573 ‘Physics of the Interstellar Medium’. We also thank an anonymous referee for useful and constructive comments that helped improve this paper.
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