Add sum filter to rank filters (minor increment)

This is a small complement to rank filters,
basically this kernel computes the pixel sum inside the structuring element (like the mean but without dividing by the structuring element surface).
If sum exceeds 255(for 8bit) or 65535 (for 16bit) the result is truncated (high order bit are ignored).
So for big structuring element it is encouraged to used 16bit image.

in this PR:

- add sum kernel
- add a test function for 8bit and 16bit images

Thanks for considering this.