core: turn divecomputer list into std::vector<>

Since struct divecomputer is now fully C++ (i.e. cleans up
after itself), we can simply turn the list of divecomputers
into an std::vector<>. This makes the code quite a bit simpler,
because the first divecomputer was actually a subobject.

Yes, this makes the common case of a single divecomputer a
little bit less efficient, but it really shouldn't matter.
If it does, we can still write a special std::vector<>-
like container that keeps the first element inline.

This change makes pointers-to-divecomputers not stable.
So always access the divecomputer via its index. As
far as I can tell, most of the code already does this.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

qt-models/divesummarymodel.cpp

@@ -116,7 +116,7 @@ Stats::Stats() :
 
 static void calculateDive(struct dive *dive, Stats &stats)
 {
-	if (is_dc_planner(&dive->dc)) {
+	if (is_dc_planner(&dive->dcs[0])) {
 		stats.diveplans++;
 		return;
 	}