Add 'trip' support to UDDF export

Currently repetition groups are used to group dives into trips. This
is not really correct. Repetition groups should really represent a
group of dives where tissue loading needs to be preserved between
dives (typically the same day).

UDDF has support for trips, using the <divetrip> tags. This is a
bidirectional link between the dives and the trips.

This patch adds support for these trips. However, it leaves in the
repetition groups. Repetition groups may need further consideration.

Signed-off-by: Martin Long <martin@longhome.co.uk>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>