We were relying on ordered relationship IDs to be generated
by other applications when we open their files, but it's not
required which broke an assumption we made. Fix this by ensuring
that generated relationship IDs don't occur in the file already.
Fixes#198
These are implied values as they are the defaults in the XSD
for optional attributes/elements. However I've noticed that
I keep running into issues where Mac Excel/Word fail to open
a file unless an optional default is provided.
I'll just add them all to hopefully increase compatibility and
save painful file format debugging sessions later.