[Constraint.Lagrangian] Make class abstract and add key function

[alxbilger]

Should fix a failing unit test: https://ci.inria.fr/sofa-ci-dev/job/sofa-framework/job/master/CI_CONFIG=macos_clang,CI_PLUGINS=options,CI_TYPE=release/3799/testReport/UnitTests/

The dynamic_cast failed in:

py::cast(dynamic_cast<ConstraintSolverImpl*>(object));

This is due to the lack of key function in the class ConstraintSolverImpl

See
android/ndk#533 (comment)

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[alxbilger]

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

[fredroy]

I don't know how you found this but very well done !!! 👍

[alxbilger]

@sofa-framework/consortium-staff hotfix in 22.06?