There is a distinct difference between the way payslips compute categories and how they sum categories over periods of time.
On payslips, children of categories are included, however not if using `sum_category` over a period of time.
After this commit they behave the same by recursively computing child categories in the `sum_category` query.
In stock Odoo, summing anything in payroll rules (but most importantly rule amounts and category amounts by code), the considered payslips are referenced from their `date_from` field. However in the USA, it is in fact the `date_to` that is more important (or accounting date). A Payslip made for 2019-12-20 to 2020-01-04 should in fact be considered a '2020' payslip, and thus the summation on other '2020' payslips must find it by considering payslips `date_to`.
