Don't assume that pressures are always positive

When planning a dive, the dive could use more gas than is in the cylinder.
So getting a negative end pressure is a useful indication to the user that
there plan might not be a good one.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: track gas consumption in cylinders and samples

This commit is a little bigger than I usually prefer, but it's all
somewhat interconnected.

- we pass around the cylinders throughout the planning process and as we
  create the plan we calculate the gas consumption in every segment and
  track this both in the end pressure of the cylinder and over time in
  the samples
- because of that we no longer try to calculate the gas consumption after
  being done planning; we just use what we calculated along the way
- we also no longer add gases during the planning process - all gases
  have to come from the list of cylinders passed in (which makes sense
  as we should only use those gases that the user added in the UI or
  inherited from a the selected dive (when starting to plan with a dive
  already selected)

With this patch I think we are close do being able to move all of the
planning logic back into the planner.c code where it belongs. The one
issue that still bothers me is that we are juggling so many dive
structures and then keep copying content around. It seems like we should
be able to reduce that.

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