Correctly handle air in the deco code

We mark air with o2.permille = 0. But it's actually 20.9% O2.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
First stab at simplistic dive planning

This comes with absolutely no gui - so the plan literally needs to be
compiled into Subsurface. Not exactly a feature, but this allowed me to
focus on the planning part instead of spending time on tedious UI work.

A new menu "Planner" with entry "Test Planner" calls into the hard-coded
function in planner.c. There a simple dive plan can be constructed with
calls to plan_add_segment(&diveplan, duration, depth at the end, fO2, pO2)

Calling plan(&diveplan) does the deco calculations and creates deco stops
that keep us below the ceiling (with the GFlow/high values currently
configured). The stop levels used are defined at the top of planner.c in
the stoplevels array - there is no need to do the traditional multiples of
3m or anything like that.

The dive including the ascents and deco stops all the way to the surface
is completed and then added as simulated dive to the end of the divelist
(I guess we could automatically select it later) and can be viewed.

This is crude but shows the direction we can go with this. Envision a nice
UI that allows you to simply enter the segments and pick the desired
stops.

What is missing is the ability to give the algorithm additional gases that
it can use during the deco phase - right now it simply keeps using the
last gas used in the diveplan.

All that said, there are clear bugs here - and sadly they seem to be in
the deco calculations, as with the example given the ceiling that is
calculated makes no sense. When displayed in smooth mode it has very
strange jumps up and down that I wouldn't expect. For example with GF
35/75 (the default) the deco ceiling when looking at the simulated dive
jumps from 16m back up to 13m around 14:10 into the dive. That seems very
odd.

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