ENH: Add (not)wrapping for Iterator-like classes.

[jhlegarreta]

ITK iterator class wrapping has no use, but corresponding files with
.notwrapped extension are added with a message warning about it are added
so that developers know about this.

[jhlegarreta]

Follow-up of comment on Iterator-like classes in issue #56.

[phcerdan]

@jhlegarreta Probably doesn't matter, but maybe for consistency we can follow the existing notwrapping style for iterators:

https://github.com/InsightSoftwareConsortium/ITK/blob/master/Modules/Core/Common/wrapping/itkImageConstIteratorWithIndex.notwrapped

itk_wrap_class("itk::ImageRegionConstIteratorWithIndex")
  itk_wrap_image_filter("${WRAP_ITK_ALL_TYPES}" 1)
itk_end_wrap_class()

[jhlegarreta]

@phcerdan Yes, I was in two minds when doing this, but @thewtex recommended in #56 to add

a comment that explains this may help others in the future.

But it is highly likely that I misinterpreted what he meant. Matt, does what I did match what you expected, or else, should I change to/add the exiting convention for (not)wrapping iterators?

Thanks.

[thewtex]

It may be clearer to have the file contents only as

message(FATAL_ERROR "No use to wrapping the iterator.")

and also fix itkImageConstIteratorWithIndex.notwrapped.

[jhlegarreta]

Done. Submitted a topic to gerrit for itkImageConstIteratorWithIndex.notwrapped.