[BarycentricMappers] Fix potential division by 0

[epernod]

To avoid NaN propagation when a wrong constraint is created using barycentricMapper and CreatePointLine.

Also add a small changes in regularGrid done during debug division by 0.

fix https://github.com/sofa-framework/BeamAdapter/issues/55 (does it works across repo?)

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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]

[hugtalbot]

if it's fine for @fredroy you can set it to ready 👍