EigenSolver is missing virtual methods used by the LinearSolverConstraintCorrection of SOFA

[jnbrunet]

This issue was raised here by Jan.

When one of Caribou's linear solvers is used by a LinearSolverConstraintCorrection of SOFA, the constraints are simply not solved at all. This is because the latter is using some virtual methods that are not implemented in Caribou's EigenSolver (the base class of all Caribou's linear solvers):

1. sofa::core::behavior::LinearSolver::addJMInvJt(R, J, f)
2. sofa::core::behavior::LinearSolver::getSystemBaseMatrix()
3. sofa::core::behavior::LinearSolver::getSystemRHBaseVector()
4. sofa::core::behavior::LinearSolver::getSystemLHBaseVector()
5. sofa::core::behavior::LinearSolver::rebuildSystem()
6. sofa::core::behavior::LinearSolver::init_partial_solve() (optional?)
7. sofa::core::behavior::LinearSolver::partial_solve(m_indices, f_indices) (optional?)

I've started a branch here with a scene example that does not work, but should. It can be used to validate that this issue is resolved.

cc @hugtalbot @Sidaty1