The deco-routines used an enum to pass around the inert gas
type. Make that globally available and make it include O2.
This will be used in a future commit to generalize access
of gas fractions.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
But only functions that operate only on gases. Functions concerning
cylinders or dives remain in dive.c or are moved to equipment.c
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>