IntervalItem::refresh()

Computes the metrics for an interval and creates a temporary
ridefile locally so you don't have to.

.. previous design should have pushed this into the
   class instead of performing it in RideItem

.. now there should be no reason to create a ridefile
   during interval discovery.

src/RideFile.cpp

@@ -369,9 +369,15 @@ struct ComparePointSecs {
 
 int
 RideFile::intervalBegin(const RideFileInterval &interval) const
+{
+    return intervalBeginSecs(interval.start);
+}
+
+int
+RideFile::intervalBeginSecs(const double secs) const
 {
     RideFilePoint p;
-    p.secs = interval.start;
+    p.secs = secs;
     QVector<RideFilePoint*>::const_iterator i = std::lower_bound(
         dataPoints_.begin(), dataPoints_.end(), &p, ComparePointSecs());
     if (i == dataPoints_.end())