added vtkCGALMeshSmoothing filter

as the title says, I added the vtkCGALMeshSmoothing filter

wasn't much work, I simply copied vtkCGALShapeSmoothing and adapted it, using the example from

https://doc.cgal.org/latest/Polygon_mesh_processing/Polygon_mesh_processing_2mesh_smoothing_example_8cpp-example.html

hope I can contribute more in the future, especially hoping to find some time to look into surface reconstruction from point clouds as described here:

https://doc.cgal.org/latest/Manual/tuto_reconstruction.html#TutorialsReconstruction_algorithms