Use get_o2() and get_he() instead of accessing permilles directly

This gets rid of problems with air.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'atdotde'

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

Conflicts:
	qt-ui/diveplanner.cpp
	qt-ui/models.cpp