Set the number of O2 sensors for Poseidon MK6

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix adding gas change to a dive

Yikes this was stupid. We mixed changing the displayed_dive and the
current_dive. So we'd pass in the displayed_dive and a pointer to the
dive_computer structure of the current dc in the current dive. Oops.

This makes much more sense. And:

Fixes #738

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