Sanity check on gas percentages

O2 + He + N2 = 100%
(well, there are some other gases, but this is close enough)

Since N2 can't be negative that means we should refuse any change where
O2 + He > 100% (or o2.permille + he.permille > 1000).

Fixes #280

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