Fixing issue with wrong order of principal curvatures with II estimators

- Fixing issue with wrong order of principal curvatures with II estimators (see [Issue #842](https://github.com/DGtal-team/DGtal/issues/842)).
- Adding some tests to check this. 
- Adding some functors in IIGeometricFunctors (IIPrincipalDirectionsFunctor and IIPrincipalCurvaturesFunctor3D)
- Moving VCM/II estimators functors into functors:: namespace