reduce default popsize parameter from 170,000 to 50,000

[andrewbaxter439]

Minor tweak - reduces default population size to a more manageable 50,000 in the GUI parameters. Very unlikely to change much in almost any runs but perhaps a useful tweak to avoid things hanging if a new user wants to attempt a run.

[pbronka]

@justin-ven I think you were recommending at least a 170k - what's your take? I'm happy with reducing this to 50k by default; we would still use a larger population for any "meaningful" runs.

[andrewbaxter439]

Thanks @pbronka - I'd made this suggestion under the assumption that I had perhaps mistakenly put it up so high! If it's by design then perhaps ignore this PR. The 50,000-person setting would be overridden in all multiruns of interest anyway, but perhaps provides an easy 'sandbox mode' for new users to experiment in the GUI?

Very, very low priority in either case, so would suggest closing if not convincing.

[justin-ven]

The 170,000 starting population was broadly needed to generate sufficient observations for the care analysis. 50,000 ought to be fine for studies relating to the population in general, but it would be useful to bear in mind that this should probably be considered a "minimum" base for analysis.