Do not allow integration over Xi in Windows

extend the sample validator to check for a non-supported sample configuration

this fixes #160 (closed) (or at least prohibits its execution)

GUI/Model/Sample/SampleValidator.cpp

@@ -46,12 +46,12 @@ void SampleValidator::validateInterferences(const LayerItem* layer)
     ASSERT(layer);
 
     int iLayout = 0;
-    for (auto* layout : layer->layouts()) {
+    for (const auto* layout : layer->layouts()) {
         iLayout++;
-        if (auto* c = dynamic_cast<const Interference2DAbstractLatticeItem*>(
+        if (const auto* c = dynamic_cast<const Interference2DAbstractLatticeItem*>(
                 layout->interference().currentItem()))
             if (c->xiIntegration()) {
-                QString message =
+                const QString message =
                     QString("Interference function of layer '%1', Particle layout %2: "
                             "Integration over Xi is currently not supported "
                             "under Windows")