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.
The default _() returns unreliable translations: 
If you analyse the code,you'll see that it calls:
res = pool.get('ir.translation')._get_source(cr, SUPERUSER_ID, None, ('code','sql_constraint'), lang, source)
the _get_source on its turn just returns a matching term (trad = res and res[0] or u'')

If you want reliable translations whereby you want the report to return the terms that is in the module's i18n file and not defined somewhere else in different context, than an override of the _() is a way to do this.