Add format_datetime to Report
From http://www.tryton.org/~irclog/2018-03-08.log.html#t16:03-4
User may want to display a datetime on the report but they are always in UTC in trytond so they need to be converted to the right timezone.
So I propose to add a function 'format_datetime' similar to 'format_date' but taking a timezone (name of pytz instance). It will be responsible to convert into timezone and format it (the time format should be a parameter also).