Reorganization of `io` directory

The vtkm/io directory has been flattened. Namely, the files in vtkm/io/reader and vtkm/io/writer have been moved up into vtkm/io, with the associated changes in namespaces.

In addition, vtkm/cont/EncodePNG.h and vtkm/cont/DecodePNG.h have been moved to a more natural home in vtkm/io.