add account_trial_balance_period_xls

cf. https://github.com/luc-demeyer/account-financial-reporting/blob/7.0/account_trial_balance_period_xls/doc/doc.rst

for functional description.
I am not sure that a docstring will be more clear than the code : I override the standard "_" method by a more specific one to avoid that the wrong translation terms are returned.