[EigenBackend] fix for older versions of Eigen
requested to merge andreas.nuessing/dune-pdelab:feature/fix-eigenbackend-simplicialcholesky into master
Eigen 3.2.2 introduced a feature to change the ordering of the SimplicialCholesky method. This feature changed the signature of the SimplicialCholesky template. We now use the preprocessor to select the appropriate signature.
This fixes #56 (closed).