Implement turning XmlEntries to a document
Implement turning XmlEntries to a document

The XML entries must be placed in a single XML document. This change
adds a function which can create such a document. It will produce the
XML document as a string which can be saved to a file.

RNAcentral/rnacentral-import-pipeline,RNAcentral/rnacentral-import-pipeline,RNAcentral/rnacentral-import-pipeline,RNAcentral/rnacentral-import-pipeline