Population units

A population unit is a unit that, when built, will subtract population from the city which built it. The most well-known example of such a unit is Settlers.
Population units can have distinct population values:
* Population cost to make the unit
* Amount of population the unit will add to a city if it's allowed to join a city
* The ability to found new cities, or not.
None of the above characteristics are always present for population units. Some population units may have a population cost of 0; some may add more than +1 population to a city; some may have a cost value different from the value they add; and some many not be able to found cities at all. Check the documentation for each type of population unit for specific details.