Implement more general versions of `test_equal_ArrayHandles`, `test_equal_CellSets`, `test_equal_Fields`, and `test_equal_DataSets` functions and put them
in vtkm/cont/testing/Testing.hi with the hope that they will be useful for
others also.
DIY now depends on MPI optionally. Hence we no longer need to depend on
DIY optionally based on whether MPI was enabled. Update cmake and c++
code to always use DIY-based components.
DIY is built with MPI support if VTKm_ENABLE_MPI is ON.
