[constraint.lagrangian] Create data links in ConstraintCorrection for linear solver

[hugtalbot]

The data strings are replaced with Links! Finally!

It's obviously breaking but there is one important point: both ConstraintCorrection had a vector of LinearSolver* while in fact only one was always used. It is now back only to a single pointer.

Let's take time to discuss it with others!

@ziqiu-zeng

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[damienmarchal]

Hello @hugtalbot,

Thanks for this PR. Replacing this string by a real link was really needed move. In your comment you are saying that from the solvers list only one was used. From what I can see there are plently of "for" loops in the code that do the same operation on each solver so can you explain a bit more why they are not needed anymore ?

EDIT: the answer was given by Hugo in gitter.

[hugtalbot]

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