Command class to build a new settlement using the given unit. Upon execution this command will return an error if:
- City name is null or an empty string.
- Tile already has a settlement.
- Given unit does not have "BuildCity" property.
If no error is fired after checking these conditions BuildCityCommand will add a new settlement to unit's coordinate and remove the unit. Initial population of settlement will be
city_founding_population property of the realm.
If settlement building unit was the player's active unit this command will not find and activate player's next unit, normally active unit will be null. If needed, the command caller must make next unit active.
@author Deniz ARIKAN