[FEM.Elastic] FEMForcefield: implement computeBBox

[fredroy]

See issue

• StateAccessor and computeBBox #6077

This fixes (temporarily?) the fact that FEMForcefield was not computing its Bounding Box while rendering.

A real fix would be to fix the entire StateAccessor/MechanicalObject computeBBox design.

[with-all-tests]

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[fredroy]

[ci-build][with-all-tests]