Refactor EntityGroupOp in discrete to also handle volome.

In ModelBuilder-v3, there is a vtkModelMaterial class and its operator class,
vtkMaterialOperator. Essentially this is a partitioned group, meaning entities
can only belong to one group (in V3, the group is called Domain set, which is
normally associated with material type of attributes). In this commit, we brought
back the vtkMaterialOperator, so that users can create group of volumes (3d) or group
of faces (2d), then associate (material) attribute to them. However, ideally we
should have a univeral group op across all sessions to cluster entities, and then
these groups can be used for attribute associations.

smtk/bridge/discrete/Session.cxx

@@ -827,6 +827,14 @@ smtk::model::Group Session::addMaterialToManager(
       {
       // Add material relations and arrangements
       translated |= smtk::model::SESSION_ENTITY_ARRANGED;
+      if (material->GetNumberOfAssociations(vtkModelRegionType))
+        { // Add regions to material
+        this->addEntities(result, material->NewIterator(vtkModelRegionType), AddEntityToGroupHelper(), relDepth - 1);
+        }
+      else if(material->GetNumberOfAssociations(vtkModelFaceType))
+        { // Add faces to material group
+        this->addEntities(result, material->NewIterator(vtkModelFaceType), AddEntityToGroupHelper(), relDepth - 1);
+        }
       }
 
     // Add material properties:
@@ -1091,20 +1099,24 @@ smtk::model::Volume Session::addVolumeToManager(
   smtk::model::ManagerPtr mgr,
   int relDepth)
 {
-  if (refVolume && !mgr->findEntity(uid, false))
+  if (refVolume)
     {
-    smtk::model::Volume result(mgr->insertVolume(uid));
-    smtk::model::SessionInfoBits translated = smtk::model::SESSION_NOTHING;
-    if (relDepth >= 0)
+    smtk::model::Volume result;
+    // if there is a Volume already for refVolume, return it; otherwise, create one
+    if(mgr->findEntity(uid, false))
+      result = smtk::model::Volume(mgr, uid);
+    else
       {
-      // Add refVolume relations and arrangements
-      this->addEntities(result, refVolume->NewAdjacentModelFaceIterator(), AddRawRelationHelper(), relDepth - 1);
-      this->addEntities(result, refVolume->NewIterator(vtkModelShellUseType), AddVolumeUseToVolumeHelper(), relDepth - 1);
+      result = mgr->insertVolume(uid);
+      if (relDepth >= 0)
+        {
+        // Add refVolume relations and arrangements
+        this->addEntities(result, refVolume->NewAdjacentModelFaceIterator(), AddRawRelationHelper(), relDepth - 1);
+        this->addEntities(result, refVolume->NewIterator(vtkModelShellUseType), AddVolumeUseToVolumeHelper(), relDepth - 1);
+        }
+      this->addProperties(result, refVolume);
       }
 
-    this->addProperties(result, refVolume);
-    translated |= smtk::model::SESSION_PROPERTIES;
-
     return result;
     }
   return smtk::model::Volume();