Fix W' calculator for bad data

Fix 1s smoothing to ensure time doesn't go backwards
as when the same timestamp is used more than once the
QwtSpline used to interpolate returns all zeroes.

src/WPrime.cpp

@@ -71,9 +71,16 @@ WPrime::setRide(RideFile *input)
     // create a raw time series in the format QwtSpline wants
     QVector<QPointF> points;
     int last=0;
+    RideFilePoint *lp=NULL;
     foreach(RideFilePoint *p, input->dataPoints()) {
-        points << QPointF(p->secs, p->watts);
+
+        // lets not go backwards -- or two sampls at the same time
+        if ((lp && p->secs > lp->secs) || !lp)
+            points << QPointF(p->secs, p->watts);
+
+        // update state
         last = p->secs;
+        lp = p;
     }
 
     // Create a spline