ConvexHull: add parameter to explicitly set dimensionality of the data/hull

I had qhull 2012 via pcl 1.7.2 complaining over here that the input is,
indeed, _not_ three dimensional when computing the hull of a detected table.
Turned out pcl is always told to reconstruct a 3d-hull.
Merge pull request #31 from v4hn/convex-hull-explicit-dimensionality

ConvexHull: add parameter to explicitly set dimensionality of the data/hull