[FIX] account_voucher: display the payment_rate_currency_id field when needed
[FIX] account_voucher: post accounting entry related to a currency rate difference in all cases
[FIX] account_voucher: fixed yaml test wromgly computing the entry related to the currency difference
[FIX] account_voucher: there is no reason to not post currency difference entry at each full reconciliation, whatever the voucher and the line currencies