[FIX] account: aged partner balance, advance partial payment

Partners totals were not correct if the partner paid partially an invoice in advance
For an invoice of 20.000 in the future, with a payment made in advance of 5000
The column not due must contains 20.000, as the amount is not yet due
One of the column 1-30, 30-60, ... (accordingly on when the payment was made). must contains -5000
The total should be 15.000