cap
   cap(surface, use_center)  
   
   Fill the holes in a surface mesh with planar faces.                     
   
   Args: 
     surface (vtkPolyData): A polygonal surface. 
     use_center (bool): If true then the planar faces are constructed using   
        polygons connected to hole centers.                                   
   
   Returns (vtkPolyData): The capped geometry.                                 

cap_with_ids
   cap_with_ids(surface, fill_id=0, increment_id=True)  
   
   Fill the holes in a surface mesh with planar faces and adding data      
   arrays to the returned vtkPolyData object identifying faces.            
   
   A cell data array named 'ModelFaceID' is added used to assign an integer
   face ID to each cell (polygon) in the vtkPolyData object.               
   
   Args: 
     surface (vtkPolyData): A polygonal surface.                           
     fill_id (int): The initial face ID to use.                            
     increment_id (bool): If True then assign each face a new ID starting  
        from 'fill_id' and incremented by 1, else assign each face the     
        same ID.                                                           
   
   Returns (vtkPolyData): The capped geometry.                             

centerlines
   centerlines(surface, inlet_ids, outlet_ids, use_face_ids=False)         
   
   Compute the centerlines for a closed surface.                            
   
   Args: 
     surface (vtkPolyData): The vtkPolyData object representing a closed   
        surface. 
     inlet_ids (list[int]): The list of integer IDs identifying the vessel 
        inlet faces.                                                       
     outlet_ids (list[int]): The list of integer IDs identifying the vessel
        outlet faces. 
     use_face_ids (bool): If True then the input IDs are face IDs, else    
        they are node IDs.                                                 
   
   Returns (vtkPolyData): The centerlines geometry (lines) and data.       

distance_to_centerlines
   distance_to_centerlines(surface, centerlines)  
   
   Compute the distance beteen centerlines and surface points.             
   
   Args: 
     surface (vtkPolyData): The vtkPolyData object representing a closed   
        surface.                                                           
     centerlines (vtkPolyData): The vtkPolyData object returned from a     
        centerlines calculation. 
   
   Returns (vtkPolyData): The vtkPolyData  object of the original surface  
      with a 'DistanceToCenterlines' point data array storing the distances.