general: simplify a few unit manipulations

Now that we have defined addition and subtraction on unit
classes, let's use them in a few examples.

Yes, some of these are a bit pointless, because they are
of the kind
        a.mbar - b.mbar => (a-b).mbar

However, these probably should be further simplified
by storing the result in a unit type.

This commit is mostly a proof-of-concept.

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

core/divelogexportlogic.cpp

@@ -110,7 +110,7 @@ static void exportHTMLstatistics(const QString filename, struct htmlExportSettin
 			out << "\"MAX_TEMP\":\"" << (s.max_temp.mkelvin == 0 ? 0 : get_temperature_string(s.max_temp)) << "\",";
 			out << "},";
 			total_stats.selection_size += s.selection_size;
-			total_stats.total_time.seconds += s.total_time.seconds;
+			total_stats.total_time += s.total_time;
 		}
 		exportHTMLstatisticsTotal(out, &total_stats);
 	}