Added flat matrix and vector used for ElementMatrix and ElementVector

src/amdis/common/CMakeLists.txt

src/amdis/common/FlatMatrix.hpp

src/amdis/common/FlatVector.hpp

src/amdis/linearalgebra/DOFMatrixBase.hpp

src/amdis/linearalgebra/DOFVectorBase.hpp

test/CMakeLists.txt

test/FlatMatVecTest.cpp