prevent warning, add comment

Framework/Particles/ParticleProperties.cc

@@ -30,9 +30,10 @@ namespace corsika::particles {
   
   Code ConvertFromPDG(PDGCode p) {
       static_assert(detail::conversionArray.size() % 2 == 1);
-      auto constexpr maxPDG{(detail::conversionArray.size() - 1) >> 1};
+      // this will fail, for the strange case where the maxPDG is negative...
+      unsigned int constexpr maxPDG{(detail::conversionArray.size() - 1) >> 1};
       auto k = static_cast<PDGCodeType>(p);
-      if (abs(k) <= maxPDG) {
+      if ((unsigned int)abs(k) <= maxPDG) {
           return detail::conversionArray[k + maxPDG];
       } else {
           return detail::conversionMap.at(p);