added ability to get gmm weight

src/main/java/gov/usgs/earthquake/nshmp/model/GmmTree.java

 package gov.usgs.earthquake.nshmp.model;
 
+import static java.util.stream.Collectors.toUnmodifiableMap;
+
+import java.util.Map;
 import java.util.Optional;
 import java.util.OptionalDouble;
 import java.util.Set;
-import java.util.stream.Collectors;
 
 import com.google.gson.JsonElement;
 
@@ -37,7 +39,7 @@ public class GmmTree {
    */
 
   private final LogicTree<Gmm> tree;
-  private final Set<Gmm> gmms;
+  private final Map<Gmm, Double> gmms;
 
   /* GmmConfig fields. */
   private final OptionalDouble maxDistance;
@@ -45,8 +47,9 @@ public class GmmTree {
   private GmmTree(LogicTree<Gmm> tree, Optional<GmmConfig> config) {
     this.tree = tree;
     this.gmms = tree.stream()
-        .map(Branch::value)
-        .collect(Collectors.toUnmodifiableSet());
+        .collect(toUnmodifiableMap(
+            Branch::value,
+            Branch::weight));
     this.maxDistance = config.isPresent()
         ? OptionalDouble.of(config.orElseThrow().maxDistance)
         : OptionalDouble.empty();
@@ -64,7 +67,15 @@ public class GmmTree {
 
   /** The set of GMMs in the logic tree. */
   public Set<Gmm> gmms() {
-    return gmms;
+    return gmms.keySet();
+  }
+
+  /**
+   * The logic tree weight of the requested GMM.
+   * @throws NullPointerException if the supplied GMM is not in this tree
+   */
+  public double gmmWeight(Gmm gmm) {
+    return gmms.get(gmm);
   }
 
   /** The maximum applicable distance for the GMMs in the logic tree. */