Gases and events support for UDDF export

This patch generates a gases section on UDDF export. The section
consists of all the unique gas mixes sawn on the dive log.

The latter part includes events to the export. Gas change and heading
events are properly translated to respective UDDF naming. Other events
are copied over as is. Some of the events are the same (ascent, deco,
rbt, surface) in UDDF some are not available.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix pressures on UDDF export

Examples on UDDF specification indicate that pressures are reported in
Pascal. Also prefer the sample data from first dive computer for start
and end tank pressures.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix end pressure on DLD export

While working on UDDF export I noticed that the grabbing of last
sample pressure was not working properly. This will fix it for DLD
export.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>