convert a PAULA etree into an XML string.

def paula_etree_to_string(tree, dtd_filename):
    """convert a PAULA etree into an XML string."""
    return etree.tostring(
        tree, pretty_print=True, xml_declaration=True,
        encoding="UTF-8", standalone='no',
        doctype='<!DOCTYPE paula SYSTEM "{0}">'.format(dtd_filename))