[Spring] Add fixed weak constraint and examples

[bakpaul]

Add a FixedWeakConstraint. It is really a simplified version of RestShapeSpringForcefield that only fixes points with respect to their rest_shape given one single value of stiffness. I didn't use an inheritance because RestShapeSpringForcefield does too much and the two set of features aren't really compatible. Keep it simple...

This completes the holy trinity of fixed constraints, now we will have Weak, Projective and Langrangian fixed constraints. It unifies the mechanism.

[ci-depends-on https://github.com/SofaDefrost/SoftRobots/pull/312]
[ci-depends-on https://github.com/SofaDefrost/STLIB/pull/127]
[ci-depends-on https://github.com/sofa-framework/BeamAdapter/pull/160]
[ci-depends-on https://github.com/SofaDefrost/Cosserat/pull/148]
[ci-depends-on https://github.com/SofaDefrost/ModelOrderReduction/pull/146]
[ci-depends-on https://github.com/SofaDefrost/SoftRobots.Inverse/pull/47]
[ci-depends-on https://github.com/SofaDefrost/SoftRobots.DesignOptimization/pull/3]

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[hugtalbot]

A duplicate of code ?

[fredroy]

Identical description with RestShapeFF ? 👮

[bakpaul]

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

[sofabot]

[ci-depends-on] detected during build #11.

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• Merge or close Apply SOFA PR 4750 changes SofaDefrost/SoftRobots#312
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