Show the gas mixing information only when needed

Recreational divers, especially ones diving with air, do not need to be
shown that their gas can be mixed with 0 liters of O2 and He. And
similarly Nitrox diver does not need to know about 0 liters of He.

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