Add vtkDataObjectTreeToPointSet filter and use it for Merge Blocks

Using this filter adds a property to Merge Blocks to select the output type. By default, the output type is vtkUnstructuredGrid, as before. Changing the output type to vtkPolyData will make the filter produce a vtkPolyData from leaves in the input composite dataset that are vtkPolyData (non-polydata leaves are skipped).

In addition, use the new vtkAppendDatasets filter for Append Datasets. This change makes Append Datasets produce a vtkPolyData if all inputs are vtkPolyData, and vtkUnstructuredGrid otherwise.

Depends on vtk/vtk!5095 (merged)

Closes #18708 (closed)