message: Update fillPopulationRequirements to return results
  This is perhaps more clear than using an out parameter, and just as
  efficient.
diff: @@ -23,10 +23,12 @@ namespace
  /**
  * Fills population requirements fields in a Structure.
  */
  - void fillPopulationRequirements(PopulationPool& populationPool, const PopulationRequirements& required, PopulationRequirements& available)
  + PopulationRequirements fillPopulationRequirements(PopulationPool& populationPool, const PopulationRequirements& required)
  {
  - available[0] = std::min(required[0], populationPool.availableWorkers());
  - available[1] = std::min(required[1], populationPool.availableScientists());
  + return {
  + std::min(required[0], populationPool.availableWorkers()),
  + std::min(required[1], populationPool.availableScientists())
  + };
  }
  @@ -300,7 +302,7 @@ void StructureManager::updateStructures(const StorableResources& resources, Popu
  const auto& populationRequired = structure->populationRequirements();
  auto& populationAvailable = structure->populationAvailable();
  - fillPopulationRequirements(population, populationRequired, populationAvailable);
  + populationAvailable = fillPopulationRequirements(population, populationRequired);
  if ((populationAvailable[0] < populationRequired[0]) ||
  (populationAvailable[1] < populationRequired[1]))
  