Add cell_filter_kwargs to relax method that is passed to ExpCellFilter

[yuuukuma]

Summary

Add cell_filter_kwargs to relax method that is passed to ExpCellFilter
This extension allows us to e.g. relax lattice constant along particular directions, e.g.,

structure.relax(cell_filter_kwargs={"mask": (1,1,0,0,0,1)}))

Then, the lattice is optimized only in xy-plane.

Because the change is tiny, I didn't add a unittest for it.

Checklist

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• Tests have been added for any new functionality or bug fixes.
• All linting and tests pass.

[janosh]

@yuuukuma You deleted the file test_files/vasprun.xml.dfpt.ionic which is causing a test to fail. Please restore it. Also, could you add/modify a test that for the new cell_filter_kwargs?