prep FRA definition for OECD

[cjacobscrisioni]

• [x] loosen centres definition in catchment generation
• [x] pop thresholds in thousands, not millions
• [ ] include FUAs
• [ ] store information on the biggest settlement in the resulting fras (include in every step?)
• [ ] create some buttons or share batches
• [x] share created networks (to avoid mem size limitations at OECD)
• [ ] documentation