Rejecting “heads on beds” and “income tax dependent” approaches, “economic unit” is proper method to determine household size and fractional children are considered. “[Section] 707(b) means test calculation will be affected by the threshold determination of how many people are part of her ‘household,’ as determined for purposes of § 1325(b). . . . [I]t is not immediately clear how to define ‘family’ even as part of the determination of a ‘household.’ . . . Because the term ‘household’ ‘lends itself to more than one reasonable interpretation,’ it is ambiguous. . . . [N]othing in § 1325(b) directly or indirectly incorporates the Census Bureau’s definition of ‘household.’ . . . [T]he heads-on-beds approach . . . is wholly unrelated to any bankruptcy purpose and does not serve the Code’s objective of identifying a debtor’s deductible monthly expenses and, ultimately, his or her disposable income. . . . It makes little sense to allow debtors to broadly define their ‘households’ so as to include individuals who have no actual financial impact on the debtor’s expenses. . . . [U]nlike the heads-on-beds approach, the economic unit approach is consistent with § 1325(b), the BAPCPA, and the Code as a whole. By examining the financial interdependence of individuals to determine whether someone is an economic part of the debtor’s household, bankruptcy courts are able to avoid over- and under-inclusive results that would result by artificially defining ‘household’ according to factors unrelated to which individuals within a residence impact the debtor’s financial situation. . . . Under this method, a debtor’s ‘household’ would include individuals who operate as an ‘economic unit’ with the debtor . . . . In contrast to the ‘economic unit’ approach, the income tax dependent approach fails to match the goals of the BAPCPA and the Code. . . . [T]he income tax dependent approach tends to be under-inclusive for purposes of ascertaining a debtor’s household size and disposable income. . . . [D]ividing individuals into fractional members of a household is less than ideal. At the same time, we recognize that the Debtor’s situation is increasingly common in modern American life, and that the number of individuals with a financial relationship to a debtor may well vary depending on the day of the week and other circumstances.”