Compute district scaling factors

[tdm32]

With demography module argument "equal_allocation_by_district" we need to also compute the scaling factors by district. This PR calculates those district-level scaling factors and writes them to the demography logger and to tlo.methods.population logger.

Notes:

• if equal_allocation_by_district=False, the district-level scaling factors are not logged
• the bed days scaled capacity and capabilities_coefficient uses the original scaling factor and doesn't scale by district

Next steps:

• need to update extract_results function to use the district_level scaling factors where appropriate
• also calculations of deaths and DALYs need to be scaled by district