Scale the weight window cutoff with the window normalization

WeightWindow::scale multiplies the lower, upper, and survival weights by a
factor but left weight_cutoff fixed, so after a renormalization the cutoff no
longer sat at the same relative depth below the window. Scale weight_cutoff by
the same factor so it keeps its relative position in any normalization frame.

Split out of #3968 for focused review.

Co-authored-by: jon-proxima <jon@proximafusion.com>
Co-authored-by: John Tramm <jtramm@gmail.com>

Author: 3 people

include/openmc/weight_windows.h

@@ -61,6 +61,9 @@ struct WeightWindow {
     lower_weight *= factor;
     upper_weight *= factor;
     survival_weight *= factor;
+    // The cutoff is scaled with the window bounds so that it sits at the same
+    // relative depth below the window in any normalization frame
+    weight_cutoff *= factor;
   }
 };