Add helper computeMinutesUntilCarryOverEnd().

Used in new tariff-calculator. Replace of previous wrong approach of
        using m_range.duration.

library/include/mobilisis/tariff_carryover.h

@@ -61,6 +61,28 @@ struct ATBTariffCarryOver {
         return QString("ERROR unknown carry over application: %1").arg(static_cast<int>(m_carryOverIf));
     }
 
+    int computeMinutesUntilCarryOverEnd(QDateTime const &dt) {
+        int minutes = 0;
+        QString end = m_range.m_end.toString(Qt::ISODate);
+        if (end == "24:00:00") {
+            QDateTime t = dt.addDays(1);
+            t.setTime(QTime(0,0,0));
+            minutes = dt.secsTo(t) / 60;
+        } else {
+            QDateTime t(dt);
+            t.setTime(QTime::fromString(end, Qt::ISODate));
+            minutes = dt.secsTo(t) / 60;
+        }
+
+        if (minutes < 0 || minutes > m_range.m_duration) {
+            minutes = 0;
+        }
+
+        // qCritical() << __func__ << ":" << __LINE__ << "minutes" << minutes;
+
+        return minutes;
+    }
+
     friend QDebug operator<<(QDebug debug, ATBTariffCarryOver const &co) {
         QDebugStateSaver saver(debug);