Add ArrayHandleIndex

Kenneth Morelandrequested to merge
kmorel/vtk-m:index-array-handle into master

Add an ArrayHandleIndex class that echoes the index. This is really a special version of ArrayHandleCounting that is of type vtkm::Id and starts at 0, but it is the most common case and does not need the extra addition.

Also modified ArrayHandleCounting to support a step, which makes the use a bit more powerful.

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