When merging dives, match up the cylinders to each other using gasmix

.. so that different computers that have different ordering of the same
cylinders will see the end result the same way.

This also fixes up the sample sensor index and generates special initial
tank change events for the dive computers that had their cylinder
indexes renamed on them.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix get_cylinder_index() gasmix comparisons

get_cylinder_index() looks up which cylinder to use based on the
gaschange event that describes the mix.  However, it was both buggy and
not very good.

It was buggy because it didn't understand about our air rules, and it
was not very good because it required an exact match (after rounding our
permille-based numbers to percent).

So fix it to use the right permille values, and look for a closest match
(using the normal sum-of-squares distance function - although I wonder
if we should consider helium percentages to be "more important" and give
them a stronger weight).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Use the new get_o2()/get_he() helper functions more widely

They do the "02=0 means air" thing autmatically, and make for less
typing.  So use them more widely in places that looked up the o2 and he
permille values of a gasmix.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix cylinder pressure interpolation

The pressure interpolation code mostly worked for the simple cases, but
got terminally confused for some more complex gas change situations,
resulting in nonsensical interpolations.

This simplifies and clarifies the code a bit, and in the process fixes a
few special cases where the gas interpolation segments didn't end up
having the end conditions set.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>