Add DB field for SEPA File Format Variant

The field is stored with the creditor. It is implemented as a
pseudoconstant (option group), currently with two options for the
standard format and for the German Sonderwurst.
Add configuration UI for SEPA File Format Variant

Part of Creditor configuration of SDD payment processors.
Emit XML header only when file generation is complete

Make sure the header is only emitted once the file was generated
successfully. Otherwise, errors during file generation won't be
displayed properly.
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.
Merge pull request #81 from sfe-ev/sonderwurst

Wunderschön! Handle German Sonderwurst in SEPA XML files.
Looking at the doc, we will need to have the same "trick" applied for other countries. It seems they are as many standards as they are countries ;)