Improve households HPA expectations with exponential average of annual HPA

[adrian-carro]

Replace current longTermHousePriceAppreciation field (and getLongTermHPA method) within both regionalHousingMarketStats and housingMarketStats in order to implement an exponential moving average. Thus, the current parameter HPA_YEARS_TO_CHECK would also need to be replaced with the corresponding smoothing parameter of the exponential average.