Correlation between appliance ownership

[cprotopa]

When a household is created, the set of present appliances is defined based on the national rate of ownership per appliance. However, this can sometimes result in cases where, for example, there is no Refrigerator (ownership 0.43) nor Fridge Freezer (ownership 0.65). I can imagine in reality a household should have exactly one of the two. Thus, it would be nice if the code can incorporate such correlation between appliances.