ADD: First pass at full implementation of new passenger generation code. Still needs work and calibration. Features:
	- Passengers/mail packets are generated globally, not city by city
	- Passengers generated only in residential houses
	- Passengers head for commercial/industrial buildings (that is, both "industries" and industrial city buildings) (TODO: Allow some passengers to head for other residential areas)
	- Mail is generated in all building types
	- Replace local/midrange/longdistance passengers with commuters and visitors, with no distance restrictions, but different journey time tolerances
	- The current "target factories" system is replaced by and subsumed into this system.
	- Resulting code simplification in many areas (TODO: Remove depracated code)
TODO: Add various simuconf.tab customisations.
TODO: Allow onward as well as return journeys
TODO: Add feature to limit number of times commuting trips can be made to buildings in any given month based on the number of jobs that they provide (conceptually tricky - needs further thought, but important).
TODO: Store the private car ownership statistics in the world, not every city
TODO: Allow more than 15 alternative destinations, and allow number of alternative destinations to vary dynamically with map size