Implement the rbt sample

The name of the RBT (Remaining Bottom Time) sample was taken from the
Uwatec dive computers. The actual definition depends on the dive
computer, but it usually corresponds to the air time remaining (with or
without some additional factors taken into account).

src/suunto_eonsteel_parser.c

@@ -551,7 +551,8 @@ static void sample_gastime(struct sample_data *info, short gastime)
 	if (gastime < 0)
 		return;
 
-	// Hmm. We have no good way to report airtime remaining
+	sample.rbt = gastime / 60;
+	if (info->callback) info->callback (DC_SAMPLE_RBT, sample, info->userdata);
 }
 
 /*