Encapsulate resource generating paraview pipeline sources.

SMTK resources are represented in ParaView pipelines as the result of a vtkSMTKSource algorithm. Internally, vtkSMTKSource contains a resource generating class (vtkSMTKResourceGenerator) whose job is to construct a resource and create a vtkMultiBlockDataSet from the resource. The resulting resource is then manipulated in situ by SMTK operations; these operations flag the source (vtkSMTKSource) for update. By encapsulating the resource generator within vtkSMTKSource, we can trigger an execution of the visualizaion pipeline without affecting a regeneration of the resource. The resource will be regenerated only if the exposed methods of the resource generator (e.g. FileName) are modified. This design is based off of ParaView's vtkMetaReader.