planner tank handling

[Dirk Hohndel]

I've been thinking about this... the planner could do a much better job
tracking tanks and gases. Right now this is a mess, left over from the old
planner code written for Subsurface 3.

Instead of having create_dive_from_plan re-create the cylinders as it
parses the plan, it should use the cylinders that were added in the UI.
And then as it walks the plan, it should update the samples to include
cylinderpressure values for each segment, based on the SAC rate that was
set (and yes, this includes adding samples at time - 1 before every gas
switch.

This requires a bit more tracking of data but at the same time should get
rid of quite a bit of awkward code that's there today and will give very
pleasing visualization of gas use during planning. If done right it will
even respond to changes to the cylinder type in the plan, etc.

Robert, does that sound reasonable?
Is this something you'd like to tackle? Or should I add this to my TODO
list?

Thanks

/D