CHANGE: Allow jobs, population, visitor demand and mail production/demand to be set individually in the .dat files of all buildings, including factories.
	- Use "population_and_visitor_demand_capacity" for specifying the population of a residential building or the visitor demand of any other building
	- Use "employment_capacity" for specifying the number of jobs in any building
	- Use "mail_demand_and_production_capacity" for specifying the number of mail packets demanded/generated in the designated period (will be 24 hours by default when settings are finalised)
	- The .dat file keywords "passenger_demand" and "mail_demand" previously used in factories now have the same meaning as "employment_capacity" and "mail_demand_and_production_capacity" respectively
	- Where these values are not specified, the original "level" is used. This can be customised by simuconf.tab settings: population_per_level, visitor_demand_per_level, jobs_per_level and mail_per_level.
	- NOTE: Monuments and houses will always have 0 jobs; monuments will always have 0 mail demand/supply