Use symbolic constants for the model numbers.
Fix the dive mode detection for the Mares Nemo Air.

For the Nemo Air, a dive mode with the value 2 indicates gauge mode
instead of freedive mode. With this change, all models from the puck
family now use the same values.
Ignore disabled gas mixes.

Gas mixes that have been disabled are stored as a mix with 0% oxygen and
0% helium. This is clearly an invalid gas mix, and it makes no sense to
return it back to the application.