[Belgian SEPA]: Fixed incorrect fields

Moved the BtchBookg node to the correct position and introduced the missing Issr node.

http://docs.oracle.com/cd/E16582_01/doc.91/e15104/fields_sepa_pay_file_appx.htm
https://www.ing.be/SiteCollectionDocuments/XMLGuide_ECT_EN.pdf

Please use 4 space indentations.