[FEM] Refactor templates in latest FEM classes

[alxbilger]

The latest FEM classes defined custom names and custom template names so that the class is registered in the factory with this custom name. However, this is not a good practice. This PR restores the regular pattern where the class is registered using its own class. Because of this, I could also remove the Element prefix on all the FEM classes.

Before:

<PrismFEMMass/>

After:

<FEMMass template="Vec3,Prism"/>

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
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• 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]

Conflicts to be solved @alxbilger
Could you also add a short description please?