Merge branch 'feature/petsc_backend' into 'master'

README.md

cmake/modules/FindPETSc.cmake

src/amdis/LinearAlgebra.hpp

src/amdis/linearalgebra/CMakeLists.txt

src/amdis/linearalgebra/petsc/CMakeLists.txt

src/amdis/linearalgebra/petsc/Communication.hpp

src/amdis/linearalgebra/petsc/Constraints.hpp

src/amdis/linearalgebra/petsc/Interface.hpp

src/amdis/linearalgebra/petsc/MatrixBackend.hpp

src/amdis/linearalgebra/petsc/MatrixNnzStructure.hpp

src/amdis/linearalgebra/petsc/MatrixNnzStructure.inc.hpp

src/amdis/linearalgebra/petsc/PetscRunner.hpp

src/amdis/linearalgebra/petsc/SolverCreator.hpp

src/amdis/linearalgebra/petsc/Traits.hpp

src/amdis/linearalgebra/petsc/VectorBackend.hpp

test/CMakeLists.txt

test/PETScCommTest.cpp