Integral invariant

- Improvement of Integral Invariant curvature estimators (both 2D / 3D & DigitalSurfaceConvolver) & tests
- Adding a functor on digital points from a predicate on digital point and a domain
- Correction of a problem with ScellToMidPoint, who don't give the exact position (must shift all coordinates by -(1/2) )

I still have a bug with the Viewer 3D in II curvature estimators example. I'm still working on it.