BUG: Defined equality and inequality operators.

Defined equality and inequality operators in
itkNthElementPixelAccessor.h.

Added tests for itkNthElementPixelAccessor.

Change-Id: Ib13ea5a1b9a8bff5e98503a84914582d96c7c984
Merge topic 'hotfix'

535aebb BUG: Defined equality and inequality operators.