Fix ATM-vs-bar confusion

SAC should be calculated in relationship to surface pressure, not "1 bar".

I also realize that we have a few other cases where we do the same
mistake: the partial pressure calculations do things like

    po2 = o2 / 1000.0 * depth_to_mbar(sample->depth.mm, dive);

which is wrong as well - the partial pressure is also relative to
standard atmospheric pressures.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix variable scope issue

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