Pass SEPA File Format Variant to templates

Note that the creditor records -- where the Format is stored -- are not
directly associated with the file entity, but rather with the individual
transaction groups making up the file; so we have to extract them at
group level -- however, we also need to ensure consistency between the
groups. (Obviously we can't mix different formats in one file...)
Implement conditionals for German Sonderwurst in SEPA file templates

We use Smarty conditionals within the standard templates rather than
separate template files, as the differences are minor, and this should
be much easier to maintain.