non-local-means: back to the beginning - windowed implementation with loops swapped
non-local-means: remove erroneous square operation
non-local-means: denoise noisy image instead of original one
non-local-means: several bug fixes, still not convinced with the results

* Fixed computation of Gaussian kernel. Now I am using the version for
  two dimensional images.

* Fixed computation of noise. Now it is Additive White Gaussian Noise,
  which can be configured using a global constant.
non-local-means: WIP still trying several alternatives for the loop swapping version