Linear-Algebra backend

The is a backend for Eigen3 implemented and for a minimal own data-structure. This needs to be extended for more libraries, e.g. MTL4 and PETSc. For PETSc already a basic implementation is available (for the old dec-pde library) that probably can easily be adapted.

There are currently just a few requirements for the backend. Basically it must be possible to run the CG or BCGS method with the implemented backend on the iterative solvers in linear_algebra/krylov.

The backend interface should be documented somewhere.