Add "legacy" IO support for cell grids.
This simply packs cell-grid JSON data into a "legacy"-like file so that ParaView can transfer geometry from server to client (when dataset size is small).
Backport: paraview/release
This simply packs cell-grid JSON data into a "legacy"-like file so that ParaView can transfer geometry from server to client (when dataset size is small).
Backport: paraview/release