Don't list gases that weren't used in profile print

If the divecomputer adds additional, unused cylinders to the dive, they
would be listed in the profile based printouts. Given that the field is
named "Gas used" that seems wrong.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't show a SAC of 0.0l/min (or cuft/min)

When we have no data about the gas consumption it makes no sense to show a
SAC of 0. Instead we should show either "unknown" or nothing.

Fixes #693

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