Planner: start the ascend with the strongest allowed deco mix.

In analyze_gaslist() we were only keeping gases that had their .depth
above the current depth, i.e. where we could switch to in the future. Now
we take note as well of the strongest gas that we could have already
switched to and switch to it before we attempt to ascend.

[Dirk Hohndel: minor whitespace and code cleanup]

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: make sure all cylinders have a sane depth

When copying cylinders from an existing dive, the mod of the cylinder
might not be set. Assume a conservative 1.4 pO2 and fill in those data.

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