Python3 fixes for structure volume calculation

When the CalculateVolume function in dvhdata.py was called, there was a
call to the itervalues() method, which is not used in Python 3. This was
changed to use the itervalues method in the six library.  Additionally,
the dictionary for each contour slice now has the key 'data' instead of
the previous 'contourData' key.

dicompyler/dvhdata.py

@@ -10,6 +10,7 @@
 # It's assumed that the reference (prescription) dose is in cGy.
 
 import numpy as np
+from six import itervalues
 
 class DVH:
     """Processes the dose volume histogram from DICOM DVH data."""
@@ -51,7 +52,7 @@ def CalculateVolume(structure):
 
     n = 0
     # Iterate over each plane in the structure
-    for sPlane in sPlanes.itervalues():
+    for sPlane in itervalues(sPlanes):
 
         # Calculate the area for each contour in the current plane
         contours = []
@@ -61,7 +62,7 @@ def CalculateVolume(structure):
             # Create arrays for the x,y coordinate pair for the triangulation
             x = []
             y = []
-            for point in contour['contourData']:
+            for point in contour['data']:
                 x.append(point[0])
                 y.append(point[1])
 
@@ -70,7 +71,7 @@ def CalculateVolume(structure):
             for i in range(0, len(x)-1):
                 cArea = cArea + x[i]*y[i+1] - x[i+1]*y[i]
             cArea = abs(cArea / 2)
-            contours.append({'area':cArea, 'data':contour['contourData']})
+            contours.append({'area':cArea, 'data':contour['data']})
 
             # Determine which contour is the largest
             if (cArea > largest):