VDBObjectTest : Tweak for OpenVDB 10.1

The grid value class bindings are intended to provide everything both as an item (`["value"]`) and as an attribute (`.value`), but in OpenVDB 10.1 the item bindings seem broken, so we must use the attribute instead.

Also added a missing import, and used `next()` to get the first element from `citerAllValues()`, knockin 2s off the runtime of the test.

Author: johnhaddon

test/IECoreVDB/VDBObjectTest.py

@@ -35,6 +35,7 @@
 ##########################################################################
 
 import os
+import unittest
 import imath
 
 import IECore
@@ -240,10 +241,10 @@ def incValue( value ) :
 
 		d = smoke.findGrid( "density" )
 
-		d2Value = list( d2.citerAllValues() )[0]
-		dValue = list( d.citerAllValues() )[0]
+		d2Value = next( d2.citerAllValues() )
+		dValue = next( d.citerAllValues() )
 
-		self.assertEqual( d2Value['value'], dValue['value'] + 1 )
+		self.assertEqual( d2Value.value, dValue.value + 1 )
 
 		# we've requested mutable grids from both vdb objects so they could have been edited.
 		self.assertFalse( smoke.unmodifiedFromFile() )