Merge branch 'feature/install-ldl-spqr' into 'master'

[CMake] install ldl.hh and spqr.hh

This will install the new headers ldl.hh and spqr.hh that were introduced by merge request !8.

See merge request !20

dune/istl/CMakeLists.txt

@@ -16,6 +16,7 @@ install(FILES
    ilusubdomainsolver.hh
    io.hh
    istlexception.hh
+   ldl.hh
    matrix.hh
    matrixindexset.hh
    matrixmarket.hh
@@ -39,6 +40,7 @@ install(FILES
    solver.hh
    solvers.hh
    solvertype.hh
+   spqr.hh
    superlu.hh
    supermatrix.hh
    umfpack.hh