ENH: Mean shift. Changing internal image calculation in BeforeThreadedGenerateData to a functor filter
for multithreading support.

ENH: Mean Shift. Extending input region for each thread to the RequestedRegion -> avoids block artefacts
at thread output regions boundaries.

ENH: Mean shift. VariableSizeVector's element-wise operators caused performance hit -> Changed to loops
over the vector dimension.

Corrected image region parameter in CalculateMeanShiftVector

Changed pixel neighborhood array to image region iterator.
Updated image border handling.
Used variable length vectors to represent joint pixel.

REFAC: Modified Mean Shift filter template parameters (order, default values). Simplified filter parameters.
Modified tests accordingly. Erased/commented out unused code.

ENH: new mean shift filter neighborhood rejection policy is coherent with edison non optimized mean shift filtering. Code cleanup must be done.

GetIterationOutput(). Updated tests accordingly.