Allocate threading equal amount of work

[claireh93]

At the moment some threads are given computationally intensive areas like cities, whereas others have predominantly sea or low population densities. Instead, we could rank grid cells by population size (as a lookup stored in the EpiSystem) and assign each thread every nth grid cell, for n total threads, distributing the work roughly evenly.

[github-actions[bot]]

Heads up @claireh93 @richardreeve @jwscook @sdl1 @wytbella - the "Simulation.jl" label was applied to this issue.