DistributionEditor: set lower bound for mean spinbox

This MR resolves #473 (closed)

The mean value of wavelength distributions cannot be set to less than the value double::min anymore in the DistributionEditor.

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::lowerLimited(std::numeric_limits<double>::min()));
     }
 
     newDistribution->initDistribution(meanVal);