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