Reenable the `vtkm::filter::CellMetric` class and
`vtkm::filter::MetricNames` static variable for backward compatibility.
(Both are of course marked deprecated.)
Also, adjust names in the new `vtkm::filter::mesh_info::CellMetric` to
conform with the rest of VTK-m style for scoped enums.
The testing files (even the headers) are not available if
`VTKm_ENABLE_TESTING` is off.
Mostly, the testing was used to generate example data sets. Instead,
change the examples to load data files.
