Continous saving tile callback now writes exr images tile by tile.
Reverted to simpler version.
Merge pull request #573 from est77/progressive_tile_saving

Continous saving tile callback now writes exr images tile by tile.
added forward declaration.
removed STANN library, now checking correctness of knn query code against a naive algorithm.

* added a new correctness check.
* small refactorings.
issue #571: not a bug, but the code deserves a comment and can be simplified.