Add normalization to max-product, avoiding underflow.

gtsam/discrete/DiscreteFactorGraph.cpp

@@ -121,6 +121,11 @@ namespace gtsam {
     for (auto&& factor : factors) product = (*factor) * product;
     gttoc(product);
 
+    // Normalize the product factor to prevent underflow
+    Ordering ordering = Ordering::Colamd(DiscreteFactorGraph{product});
+    DecisionTreeFactor::shared_ptr normalization = product.sum(ordering);
+    product = product / (*normalization);
+
     // max out frontals, this is the factor on the separator
     gttic(max);
     DecisionTreeFactor::shared_ptr max = product.max(frontalKeys);