[ODESolver] Explicit link to linear solver

[alxbilger]

It basically removes the use of MultiMatrix which hides the use of a LinearSolver. Implicitly, the first linear solver found was used. Now, the LinearSolver can be defined directly in the ODE solver as a Link.

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[alxbilger]

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

[alxbilger]

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

[bakpaul]

Why add a diamond inheritance instead of modifying directly the common parent class ODESolver ?

[alxbilger]

Because explicit solvers do not necessarily require a linear solver