removed filterSalesInfo() and changed it into getSalesInGivenPeriod() in preprocessing.R; edited cumsumMonthlyMargin()
edited getBasicDealInformation() to ensure numeric rebates values
added marginPerCustomerPerCategoryPerDeal() and related functions; deleted calculateMarginEachDeal()
removed monthDifference() and related functions to preprocessing.R; simplyfied sorting data using sortCohorts()