Little hack for htg supercursors

Super cursors in htg fail when reaching a neighbor
outside extent. This commit adds a new invalid id in vtkHyperTreeGrid
which is use to notify this case. This might need to be done better
later

(cherry picked from commit 2878eda0)

Common/DataModel/vtkHyperTreeGrid.h

@@ -105,6 +105,12 @@ public:
   vtkTypeMacro(vtkHyperTreeGrid, vtkDataObject);
   void PrintSelf(ostream& os, vtkIndent indent) override;
 
+  /**
+   * Invalid index that is returned for undefined nodes, for example for nodes that are out of
+   * bounds (they can exist with the super cursors).
+   */
+  static constexpr vtkIdType InvalidIndex = ~0;
+
   /**
    * Set/Get mode squeeze
    */

Common/DataModel/vtkHyperTreeGridGeometryEntry.cxx

@@ -21,6 +21,16 @@ PURPOSE.  See the above copyright notice for more information.
 
 #include <cassert>
 
+//-----------------------------------------------------------------------------
+vtkHyperTreeGridGeometryEntry::vtkHyperTreeGridGeometryEntry()
+{
+  this->Index = 0;
+  for (unsigned int d = 0; d < 3; ++d)
+  {
+    this->Origin[d] = 0.;
+  }
+}
+
 //-----------------------------------------------------------------------------
 void vtkHyperTreeGridGeometryEntry::PrintSelf(ostream& os, vtkIndent indent)
 {

Common/DataModel/vtkHyperTreeGridGeometryEntry.h

@@ -56,14 +56,7 @@ public:
   /**
    * Constructor
    */
-  vtkHyperTreeGridGeometryEntry()
-  {
-    this->Index = 0;
-    for (unsigned int d = 0; d < 3; ++d)
-    {
-      this->Origin[d] = 0.;
-    }
-  }
+  vtkHyperTreeGridGeometryEntry();
 
   /**
    * Constructor

Common/DataModel/vtkHyperTreeGridGeometryLevelEntry.cxx

@@ -56,8 +56,8 @@ vtkHyperTree* vtkHyperTreeGridGeometryLevelEntry::Initialize(
 //-----------------------------------------------------------------------------
 vtkIdType vtkHyperTreeGridGeometryLevelEntry::GetGlobalNodeIndex() const
 {
-  assert("pre: not_tree" && this->Tree);
-  return this->Tree->GetGlobalIndexFromLocal(this->Index);
+  return this->Tree ? this->Tree->GetGlobalIndexFromLocal(this->Index)
+                    : vtkHyperTreeGrid::InvalidIndex;
 }
 
 //-----------------------------------------------------------------------------