comment about filling in nans

Former-commit-id: 5ff124f98b77b49da253db5a6a23c2c586f91737

include/igl/swept_volume_signed_distance.cpp

@@ -87,6 +87,7 @@ IGL_INLINE void igl::swept_volume_signed_distance(
   }else
   {
 #ifndef NDEBUG
+    // Check for nans
     for_each(S.data(),S.data()+S.size(),[](const double s){assert(s==s);});
 #endif
   }