Don't copy the current dive ontop of editedDive after edit has started

If you switch to edit mode by making a change to cylinders or weights,
then the data in the editedDive was changed and afterwards the current
dive was once again copied into the editedDive - even though we already
did that earlier when setting up the display.

Fixes #539

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Correct pO2 formula in CNS calculation.

In commit e58f54cac1d0 ("Fix ATM-vs-bar confusion") Linus broke the CNS
calculations; The partial pressure is fraction of gas times ambient
pressure, not divided by.

[Dirk Hohndel: identical patch sent by Robert and Anton; I combined the
               commit messages and added the ACK from the perpetrator]

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Anton Lundin <glance@acc.umu.se>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>