Turn dive depth, temperature and duration into xml attributes

Turn dive depth, temperature and duration into xml attributes

This makes the xml save-file look way nicer: it's both smaller and
better organized.  Using individual xml nodes for random small details
is silly.

The duration even parses exactly the same, because it still ends up
being '.depth.duration' (now it's the 'duration' attribute of the dive
node, it used to be the 'duration' child node of the dive node).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>