Export additional tanks to divelogs.de

All recorded cylinders are exported to divelogs.de. This potentially
includes cylinders that have not been used.

The start and end pressure for additional cylinders is taken from the
cylinder information. Main tank gets this information from first and
last pressure reading or from the cylinder information if samples are
not available. This should be ok when there is only one cylinder with
pressure sensor, but does not probably work for people having multiple
sensors.

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