Version ambiguity in Document: fix

In general the Doc version is writable/readable now.

However there are some doubts about the _xml_version attribute that was mixed with the doc.version.  This attribute is excluded in this change as is never used. In case one needs to keep somehow the XML writer/reader version, a separate attribute should be introduced.

In total:
- removed the check on write for xml_version
- removed assignment of the _xml_version to Document to avoid side effects
- renamed args parameter to arguments so it's easier to debug
- removed check on load for xml_version