add more file extension handlers

[banesullivan]

the dataset backend here uses pyvista.read() to load files supported by PyVista/VTK. Currently these are limited to vti, vts, and vtr files but any formats the generate a vtkRectilinearGrid, vtkImageData, or vtkStructuredGrid are supported.

This dataset engine should list any file extensions supported by PyVista that will generate those data types. I'm wondering if we could have PyVista list these extensions upstream?

See https://github.com/pyvista/pyvista/blob/75d51c6c1167137051b34c7acb8286c3df8eb926/pyvista/utilities/reader.py#L2081