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.
It's not what it's doing that would be nice to be in the docstring, but **why** it's overwritten.
As in why does this `_()` definition have to be here and what's wrong with the default `_()`