Start cleaning up sensor indexing for multiple sensors

This is a very timid start at making us actually use multiple sensors
without the magical special case for just CCR oxygen tracking.

It mainly does:

 - turn the "sample->sensor" index into an array of two indexes, to
   match the pressures themselves.

 - get rid of dive->{oxygen_cylinder_index,diluent_cylinder_index},
   since a CCR dive should now simply set the sample->sensor[] indices
   correctly instead.

 - in a couple of places, start actually looping over the sensors rather
   than special-case the O2 case (although often the small "loops" are
   just unrolled, since it's just two cases.

but in many cases we still end up only covering the zero sensor case,
because the CCR O2 sensor code coverage was fairly limited.

It's entirely possible (even likely) that this migth break some existing
case: it tries to be a fairly direct ("stupid") translation of the old
code, but unlike the preparatory patch this does actually does change
some semantics.

For example, right now the git loader code assumes that if the git save
data contains a o2pressure entry, it just hardcodes the O2 sensor index
to 1.

In fact, one issue is going to simply be that our file formats do not
have that multiple sensor format, but instead had very clearly encoded
things as being the CCR O2 pressure sensor.

But this is hopefully close to usable, and I will need feedback (and
maybe test cases) from people who have existing CCR dives with pressure
data.

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

qt-models/diveplotdatamodel.cpp

@@ -40,7 +40,7 @@ QVariant DivePlotDataModel::data(const QModelIndex &index, int role) const
 		case USERENTERED:
 			return false;
 		case CYLINDERINDEX:
-			return item.cylinderindex;
+			return item.sensor[0];
 		case SENSOR_PRESSURE:
 			return item.pressure[0][0];
 		case INTERPOLATED_PRESSURE: