[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

According to the pain format .xsd's, BtchBookg should always reside in the PaymentInstructionInformation node.

In the past there was a 001.001.02 format that required the BtchBookg to reside in the Header, this is no longer the case in the newer formats.

This library claims support for 'pain.001.002.03', 'pain.001.001.03', 'pain.008.002.02', 'pain.008.001.02' which all require the BtchBookg to be in the PaymentInstructionInformation.

The other implemented child of the BaseDomBuilder, CustomerDirectDebitTransferDomBuilder never used the BatchBooking so it should continue to work.

Finally, the supported formats describe BtchBookg as 'minOccurs="0"', so omission of the field should not break functionality.