Only load required components of VTK Python lib

Also fix a bug where cell data arrays would be ignored.

python/dorie/dorie/utilities/vtktools/vtkreader.py

-import vtk
+from vtk import vtkXMLGenericDataObjectReader, \
+                vtkCellTreeLocator, \
+                vtkDataSet, \
+                vtkGenericCell, \
+                reference
 import numpy as np
 
 class VTKReader:
@@ -8,13 +12,13 @@ class VTKReader:
         self._data_array = None
 
         # set up the reader
-        self._reader = vtk.vtkXMLGenericDataObjectReader()
+        self._reader = vtkXMLGenericDataObjectReader()
         self._reader.SetFileName(file_name)
         self._reader.Update()
-        self._dataset = vtk.vtkDataSet.SafeDownCast(self._reader.GetOutput())
+        self._dataset = vtkDataSet.SafeDownCast(self._reader.GetOutput())
 
         # build the locator
-        self._locator = vtk.vtkCellTreeLocator()
+        self._locator = vtkCellTreeLocator()
         self._locator.SetDataSet(self._dataset)
         self._locator.BuildLocator()
 
@@ -22,7 +26,7 @@ class VTKReader:
         self.set_data_array(array_name, data_type)
 
         # cache for cell and ID
-        self._cell = vtk.vtkGenericCell()
+        self._cell = vtkGenericCell()
         self._cell_id = -1
 
     def set_data_array (self, array_name, data_type=None):
@@ -39,13 +43,14 @@ class VTKReader:
         if data_type == "cell" or data_type is None:
             # retrieve cell data array...
             self._data_array = retrieve_array(self._dataset.GetCellData(),
-                                            array_name)
+                                              array_name)
             self._eval = self.__eval_cell
 
-        if data_type == "vertex" or data_type is None:
+        if self._data_array is None \
+            and (data_type == "vertex" or data_type is None):
             # ...or vertex data array
             self._data_array = retrieve_array(self._dataset.GetPointData(),
-                                            array_name)
+                                              array_name)
             self._eval = self.__eval_vertex
 
         if self._data_array is None:
@@ -71,7 +76,7 @@ class VTKReader:
 
         # unused stubs
         zeros = [0, 0, 0]
-        zero_ref = vtk.reference(0)
+        zero_ref = reference(0)
 
         # interpolation weights
         weights = np.zeros(10)