Cleanup: pass gasmix by value

In a previous commit, the get_gasmix_* functions were changed to
return by value. For consistency, also pass gasmix by value.

Note that on common 64-bit platforms struct gasmix is the size
of a pointer [2 * 32 bit vs. 64 bit] and therefore uses the
same space on the stack. On 32-bit platforms, the stack use
is probably doubled, but in return a dereference is avoided.

Supporting arbitrary gas-mixes (H2, Ar, ...) will be such an
invasive change that going back to pointers is probably the
least of our worries.

This commit is a step in const-ifying input parameters (passing
by value is the ultimate way of signaling that the input parameter
will not be changed [unless there are references to said parameter]).

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

profile-widget/tankitem.h

@@ -25,7 +25,7 @@ public slots:
 	void modelDataChanged(const QModelIndex &topLeft = QModelIndex(), const QModelIndex &bottomRight = QModelIndex());
 
 private:
-	void createBar(qreal x, qreal w, struct gasmix *gas);
+	void createBar(qreal x, qreal w, struct gasmix gas);
 	DivePlotDataModel *dataModel;
 	DiveCartesianAxis *hAxis;
 	struct dive diveCylinderStore;