cleanup of headers and itl solvers in mtl backend

cmake/modules/AmdisMacros.cmake

cmake/modules/CMakeLists.txt

cmake/modules/FindHYPRE.cmake

cmake/modules/FindMTL.cmake

config.h.cmake

src/amdis/linearalgebra/mtl/CMakeLists.txt

src/amdis/linearalgebra/mtl/HyprePrecon.hpp

src/amdis/linearalgebra/mtl/ITL_Preconditioner.hpp

src/amdis/linearalgebra/mtl/ITL_Solver.hpp

src/amdis/linearalgebra/mtl/KrylovRunner.hpp

src/amdis/linearalgebra/mtl/Preconditioner.hpp

src/amdis/linearalgebra/mtl/SolverPrecon.hpp

src/amdis/linearalgebra/mtl/UmfpackRunner.hpp

src/amdis/linearalgebra/mtl/itl/CMakeLists.txt

src/amdis/linearalgebra/mtl/itl/block_diagonal.hpp

src/amdis/linearalgebra/mtl/itl/details.hpp

src/amdis/linearalgebra/mtl/itl/fgmres.hpp

src/amdis/linearalgebra/mtl/itl/fgmres_householder.hpp

src/amdis/linearalgebra/mtl/itl/gcr.hpp

src/amdis/linearalgebra/mtl/itl/gmres2.hpp