make eigen and mtl a system package

Rewrite find eigen and mtl in cmake to link against imported targets. This makes these external mackages system packages and thus compiler warning are not shown anymore. It hast to be checked which cmake version is required for this to work. And it has to be checked whether we could do the same for other external packages in dune.

added 4 commits

• 86f1bf76...0215302f - 2 commits from branch master
• c40196fe - check whether BACKEND is either MTL, EIGEN, or ISTL
• d4b0dfe1 - merge branch issue/cmake_backend_check into feature/cmake_packages

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added 1 commit

• cfaf3fd7 - added search path to mtl includes based on MTL_DIR

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enabled an automatic merge when the pipeline for cfaf3fd7 succeeds

merged

mentioned in commit 344e1aa3