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/parse-xml.cpp

@@ -678,7 +678,7 @@ static void eventtime(const char *buffer, duration_t *duration, struct parser_st
 {
 	sampletime(buffer, duration);
 	if (state->cur_sample)
-		duration->seconds += state->cur_sample->time.seconds;
+		*duration += state->cur_sample->time;
 }
 
 static void try_to_match_autogroup(const char *name, char *buf, struct parser_state *state)