Reuse generated XML header across functions

The new global variable AUTODOC_XML_HEADER unifies five occurrences
of the same XML header (previously with tiny variations).
Use Directory() to handle path_to_xmlfiles

This way, CreateAutomaticDocumentation can now also accept paths
without trailing slash. In fact, it is now possible to directly pass
an IsDirectory object to it. Thus now this parameter matches the first
parameter of MakeGAPDocDoc.