cleaning/refactoring + new class to compare the radius of a circle with a given radius
 alpha-shape computation based on a graham scan and the associated test
new class for computing alpha-shapes
 adding #ifndef in header files
 removing RadiusCirclePredicate (must use CircumcircleRadiusPredicate instead)
interface corrected (several vertices may be retrieved in a single step)
Merge branch 'master' into alphaShape
convex hull helpers update (open and closed curves)
 new test, just for alpha-shape on boundaries of half-planes
Merge branch 'alphaShape'