- either take by const-ref
- or move values

This definition avoid useless conversions, is more robust (not a macro), and
it's hidden in an anonymous namespace for now (it should be moved into a
central _toolbox_ file)

- remove unused includes
- add missing ones (in the right file)
- reorder: OTB > ITK > 3rd part > STD

Directly jump to `final` as the class won't be overridden

(latest OTB style)

- harmonize indent and use latest OTB style (Allman's indenting style)
- remove trailling spaces

This commit doesn't need much attention. It serves to avoid extra noises in
futures commits that would automatically change indenting...

and rename the files to avoid conflict

... to invalid normals, DEM values...

... to nodata, NaNs, "folded" neighbours (when left and right are a the same
position...)

To avoid clashes with otbIteratorHelpers from OTB