PERF: Adding performance oriented ScanlineIterator

Adding a new iterator without a conditional in the operator++, to
create an inner loop without branching to improve performance.

Using the new iterator with an inner loop for an individual line and
an outer loop to advance through the lines, in the Functor based
filters.

This has resulted in a 2x-3x speed up for single threaded execution of
128^3 images in filters such as CastImageFilter and
AddImageFilter. Close to 100 functor based image filters exist in ITK
all should benefit from this change.

Since the ImageRegionIterator was removed from the UnaryImageFilter,
a missing header needed to be added in a couple places.

Change-Id: I83f7f0753a271389d8bfae2eda8d6ad1848510c4
Merge topic 'ScanlineIterator'

ab29cc5 PERF: Adding performance oriented ScanlineIterator