Update pointer file

Updated the pointer file to work better with some LOCKSS requirements (though this is an independent change).

1. Add compression event and Archivematica agent
1. Move ADMID to structMap

Also did some cleanup in archivematicaXMLNamesSpace and createPointerFile, since I've learned some about lxml and namespaces since I originally wrote it. (Notably that () around an ElementMaker structure are unnecessary, hence a lot of whitespace changes)

refs #6500