[SolidMechanics.FEM] Small update on container access to avoid unecessary check

[epernod]

continue porting changes from old branches: https://github.com/epernod/sofa/pull/12/files

getTetrahedron(i) perform a test if i is not out of bounds. When a function is called often and loop on the list of tetrahedra, it is best to get a ref to the array and loop on it.

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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).

[epernod]

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