[SofaKernel] ADD brace initializer to helper::vector class

[VannesteFelix]

This PR add:
Initialization of helper::vector using {}

Example:
helper::vector< double > vec1 = {5,4,7,1,2};
helper::vector<helper::vector< double >> vec2 = {{5,4},{7,1,2}};

---

This PR:

• builds with SUCCESS for all platforms on the CI.
• does not generate new warnings nor unit test failures.
• does not break existing scenes.
• does not break API compatibility.
• has been reviewed.
• is more than 1 week old (or has fast-merge label).

Reviewers will merge only if all these checks are true.

[sofabot]

Thank you for your pull request!
Someone will soon check it and start the builds.

[damienmarchal]

Hi Felix,

Thank you for you PR. The feature look fine to me, I see no drawback.

[damienmarchal]

[ci-build]

[hugtalbot]

Hi @VannesteFelix

Thank you very much for your contribution.
Could you please add the proper labels on your PR ? Thanks !

[damienmarchal]

[ci-build]

[damienmarchal]

Done.