Use utility function instead of locally declared function

Issue: #4162

name.abuchen.portfolio.ui/src/name/abuchen/portfolio/ui/views/dataseries/StatementOfAssetsSeriesBuilder.java

@@ -1,6 +1,7 @@
 package name.abuchen.portfolio.ui.views.dataseries;
 
 import static name.abuchen.portfolio.util.ArraysUtil.accumulateAndToDouble;
+import static name.abuchen.portfolio.util.ArraysUtil.add;
 import static name.abuchen.portfolio.util.ArraysUtil.toDouble;
 
 import org.swtchart.IBarSeries;
@@ -116,16 +117,4 @@ private void addClient(DataSeries series, PerformanceIndex clientIndex)
             configure(series, barSeries);
         }
     }
-
-    private long[] add(long[] a, long[] b)
-    {
-        if (a.length != b.length)
-            throw new IllegalArgumentException("length mismatch " + a.length + " != " + b.length); //$NON-NLS-1$ //$NON-NLS-2$
-
-        long[] result = new long[a.length];
-        for (int ii = 0; ii < result.length; ii++)
-            result[ii] = a[ii] + b[ii];
-
-        return result;
-    }
 }