[Mapping.Linear] Use links instead of raw pointers

[alxbilger]

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By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

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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).

[hugtalbot]

Compilation error on CI, e.g. :

/Users/ci/Jenkins/workspace/sofa-framework/PR-4996/macos_clang_options_release/src/Sofa/Component/Mapping/Linear/src/sofa/component/mapping/linear/Mesh2PointMechanicalMapping.inl:53:54: error: use 'template' keyword to treat 'get' as a dependent template name
        l_topologicalMapping.set(this->getContext()->get<Mesh2PointTopologicalMapping>());

[epernod]

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