Correctly check if tank is in use

We always checked if the tank was in use by the planner / dive editor -
even if we were not in dive edit mode.

With this patch, when not in dive edit mode, we check our cylinder "used"
flag instead.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add test dive to show erroneous pp graphs

Add simple test dive to show the erroneous pp graph in case of
multiple periods of high p02. See commit aa0cd792bbe3e for the fix.

Signed-off-by: Jan Mulder <jlmulder@planet.nl>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't put cylinder widget in edit mode if deletion of cylinder failed

If the user tries to delete a cylinder that is in use and that action is
rejected, the cylinder widget (and the whole dive) should not be put into
edit mode. After all, nothing changed.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>