ENH: Rename nodes to points in vtkStructuredData

Modified the function names in vtkStructuredData to use "points"
instead of "nodes" in order to be consistent with the rest of VTK.
Further, updated all affected code that is using vtkStructuredData
to reflect this change.

Change-Id: Iad0a876b49e0b5439e47b963c73885c2ade3c0d6
BUGFIX: Fix issue with unigrid enzo datasets

Fixed issued with unigrid enzo datasets. The reader
was calculating number of levels to be 0, which was
leading to a segfault. Added code to detect unigrid
datasets and fixed the issue by setting the number
of levels to 1.

Change-Id: Ie9f1fbe8c3c38c69e5761b357256be9531ed10f3
Merge topic 'rename-nodes-to-points' into master

9bfa169c ENH: Rename nodes to points in vtkStructuredData
Merge topic 'enzo-unigrid-support' into master

df25648b BUGFIX: Fix issue with unigrid enzo datasets