petsc_save_checkpoint() does not include the recent XDMF changes.

[gthyagi]

petsc_save_checkpoint() does not include the recent XDMF updates for creating vertex and cell datasets; it only saves datasets based on the MeshVariable degree.
Do we need two separate functions for checkpointing (write_timestep() and petsc_save_checkpoint())?
If both are required, the recent XDMF updates should also be added to petsc_save_checkpoint().
For easier maintenance, it may be better to keep a single checkpointing function.

[lmoresi]

Fixed on development branch (commit 0f220a1).

petsc_save_checkpoint() now delegates to write_timestep(), so it gets all the recent XDMF improvements: vertex/cell compatibility groups, field projection (P2→P1), and tensor repacking for ParaView.

This is a single checkpointing function internally now — petsc_save_checkpoint() is a convenience wrapper around write_timestep().

Note: The output file layout changes from a single HDF5 file to per-variable files (same format as write_timestep()), so scripts that read back the old _step_NNNNN.h5 filenames will need updating.

@gthyagi — should we cherry-pick this to main for a v3.0.1 patch, or is it fine to wait on development for the next release?