Add PCL style shared_ptr shortcuts for the implemented registration methods

[PerceptMD]

Its just a minor thing to add and most probably not so many ppl. are using it, but to keep up the PCL implementation style all the specialized registration methods could provide the shared_ptr shortcut ::Ptr.

As an example we can take a look into the public part of pcl::gicp class:

      using Ptr = shared_ptr< GeneralizedIterativeClosestPoint<PointSource, PointTarget> >;
      using ConstPtr = shared_ptr< const GeneralizedIterativeClosestPoint<PointSource, PointTarget> >;

[koide3]

Yeah. There should be Ptr definitions. I'll add Ptr definitions.