Planner: add warning if the plan consumes more gas than provided

If we have size information including working pressure for a cylinder, we
already track the expected pressure at each way point - this way we can
also alert the user if more gas is consumed than is available in the
cylinder.

This does not include sound planning strategies like "rule of thirds". It
simply assumes that you won't be able to breathe down the cylinder past
about 10bar (using 0 as cutoff seemed silly).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: more fixes for gas tracking

Gas switches are now for the current segment, not for the next.

We can only track gas for cylinders for which we have size / working
pressure information. Print a warning for others.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: track gas used even if we don't have a real cylinder

We tracked gas used by simulating a dive with a cylinder - but for that we
need a cylinder size and working pressure. If the user just enters a gas
but no cylinder data (likely in order to figure out how much gas is used
so that she then can pick a big enough cylinder), we didn't show any gas
consumption.

It kinda sucks to add another member to the cylinder structure, but this
seemed far more reasonable then some other, global structure that
independently tracks gas usage. This just seemed to make sense.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: correctly track gas used

This part of the algorithm was off as part of it (but not all of it -
crazy, I know) assumed the old semantic for gas changes where the new gas
was introduced at the end of the segment and not at the beginning.

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