ENH: add in preparatory SimPaths single run?

[andrewbaxter439]

A suggestion for this script would be to allow for optional running/re-running of SimPathsStart to generate the input.mv.db database before multi-runs. This is necessary and so far I only know how to do it using the GUI. SimPathsStart I think should be able to run headless, but at the moment those options seem disabled and have perhaps been deprecated.

Benefits of optional re-running single run:

• Can stabilise all run parameters by modifying/archiving input files - no longer have to re-enter GUI options
• Easier to run/describe run for reproducibility
• Can all be done headless
• Can force do_full_run.sh to re-run the setup to absolutely remove all traces of previous runs and ensure correct population being used