Merge branch 'feature/eigen_backend' into 'develop'

cmake/modules/AmdisMacros.cmake

config.h.cmake

examples/ellipt.cc

examples/init/ellipt.dat.2d

src/amdis/LinearAlgebra.hpp

src/amdis/LinearSolvers.hpp

src/amdis/ProblemStat.cpp

src/amdis/ProblemStat.hpp

src/amdis/QuadratureFactory.hpp

src/amdis/linear_algebra/CMakeLists.txt

src/amdis/linear_algebra/DOFMatrixBase.hpp

src/amdis/linear_algebra/DOFVectorBase.hpp

src/amdis/linear_algebra/RunnerInterface.hpp

src/amdis/linear_algebra/eigen/CMakeLists.txt

src/amdis/linear_algebra/eigen/DOFMatrix.hpp

src/amdis/linear_algebra/eigen/DOFVector.hpp

src/amdis/linear_algebra/eigen/DirectRunner.hpp

src/amdis/linear_algebra/eigen/IterativeRunner.hpp

src/amdis/linear_algebra/eigen/PreconConfig.hpp

src/amdis/linear_algebra/eigen/SolverConfig.hpp