Added function processInterval

This function is used by addYears, addMonths, addWeeks, addDays, addHours, addMinutes & addSeconds
No more need of Dateinterval

With a minor change less code, more readable and no more need of Dateinterval
Small fix ton ensure diffinseconds truncation test
Don't need the abs() call since it does that by default.
Use the modify() for weekdays as well.