Threaded Shepard interpolation with power parameter

There are several major improvements to vtkShepardMethod. First, the algorithm has been threaded with vtkSMPTools. Second, an interpolation power parameter has been added to generalize the interpolation basis. Third, documentation has been significantly improved. Finally, in the previous version, the filter failed when a sample and interpolation point were coincident (i.e., distance between them =0); this has been fixed.