Add support for VTKHDF 2.6 "attribute data"

[nncarlson]

VTKHDF 2.6 added the (optional) specification of attributes for datasets in the Point/Cell/FieldData groups. If found, Paraview 6.1 / VTK 9.6 (?) will use this data for special purposes. For example a PointData dataset with the attribute "GlobalIds" will be used to populate the vtkGlobalPointIds array. Earlier versions looked for a dataset with the magic name "vtkGlobalPointIds".

[nncarlson]

A minimal implementation would add an optional attr character argument to the write_cell/point_data and register_cell/point_data methods. This leaves the onus to the user to use in a meaningful way. In at least some of these uses there are additional type expectations/requirements on the dataset, so an alternative (or an addition) would be to add specific methods, e.g., write_point_global_ids that enforce this.