Bug fix: update constraint optimisation path to use the correct FixSymmetry.

gelzinyterequested to merge
gelzinyte/vibes:elena/patch-fix-symmetry into master

Previously, symmetry-constrained optimisation triggered a ase.spacegroup.symmetrize.FixSymmetry import. As of ase 3.23.0 (vibes' bounds are >=3.26), the correct import is ase.constraints.FixSymmetry.

Also added a unit test for sym-constrained optimisation and updated one doc ref.

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