Create a new method on statement lines to get the currency
Use the new method for the currency in account_statement_operation_rule_dunning_fees
Add test: check if rounding rules are well skipped on multi-currency lines
Add a selection of currencies for the 'Currencies' rules

It allows to have a different setup for different currencies (the amounts won't
be the same).
Add more multi-currency tests, fix a condition preventing multi-currency to work