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>

desktop-widgets/tab-widgets/TabDiveInformation.cpp

@@ -62,7 +62,7 @@ void TabDiveInformation::updateData()
 		gaslist.append(separator); volumes.append(separator); SACs.append(separator);
 		separator = "\n";
 
-		gaslist.append(gasname(&displayed_dive.cylinder[i].gasmix));
+		gaslist.append(gasname(displayed_dive.cylinder[i].gasmix));
 		if (!gases[i].mliter)
 			continue;
 		volumes.append(get_volume_string(gases[i], true));