[Constraint.Lagrangian] Add fixed lagrangian constraint

[bakpaul]

Add a lagrangian constraint working the same way as FixedProjectiveConstraint.
Added Vec3 and Rigid3 specialization.

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[alxbilger]

Good initiative. We will have 3 different ways to fix points. 👍

[alxbilger]

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

[bakpaul]

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

[alxbilger]

If I want to be peaky, I would say that you don't provide code for instantiation for other types than Vec3Types and Rigid3Types. For example, FixedProjectiveConstraint also provides Vec2Types, Vec1Types, Vec6Types and Rigid2Types.

And if we look at Vec1Types specifically, can you explain the difference between FixedLagrangianConstraint<Vec1Types> and UniformLagrangianConstraint<Vec1Types>?

[bakpaul]

The difference lies in the fact that the UniformLagrangianConstraint is applied to all the model points. This assumption is important because thank's to this, it is able to use a block optimization when the data iterative is set to false. Here, it wouldn't be possible because we don't have the guaranty that the points ids will be contiguous.

As for the other specialization, what I did was to replicate the two specialization already available for the BilateralLagrangianConstraint. Actually I made sure to factorize as much as possible so that it would be easier for new specialization to pbe implemented. I could add them, but given that they didn't exist for the BilateralLagrangianConstraint, I don't know if they are really useful for anyone.

[bakpaul]

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