Image reader and writer structure

Kenneth Morelandrequested to merge
kmorel/vtk-m:image-reader-structure into master

Make several updates to the image readers

  • Put the code for image readers and writers in vtkm_io.
  • Updated the naming conventions to follow VTK-m
  • Change the interface to match the VTK file readers/writers
  • Simplified how pixel types are selected

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