[473-spinbox] DistributionEditor: set lower bound for mean spinbox (Closes #473)

Merging branch '473-spinbox'  into 'main'.

See merge request !1455

GUI/Model/Device/BeamWavelengthItem.cpp

@@ -31,6 +31,7 @@ void initDistribution(DistributionItem* newDistribution,
     if (auto* nd = dynamic_cast<SymmetricResolutionItem*>(newDistribution)) {
         nd->setMean(meanVal);
         nd->setMeanDecimals(4);
+        nd->mean().setLimits(RealLimits::positive());
     }
 
     newDistribution->initDistribution(meanVal);