Clean up include structure in pdelab/backend

The chaotic include structure in the backend folder has to be cleaned up:

• Move all ISTL-related includes to backend/istl
• Deprecate old files moved to backend/istl
• Add new header backend/istl.hh that includes all ISTL functionality
• Move all Eigen-related includes to backend/eigen
• Deprecate old files moved to backend/eigen
• Adjust backend/eigen.hh to include all Eigen functionality
• Move tags.hh to folder common
• Remove PETSc files, they are completely broken and need to be reimplemented from scratch anyway

I'll work on this in andreas.nuessing/dune-pdelab/feature/cleanup-backend-structure and create a WIP merge request as soon as the first commit is created.

mentioned in merge request !22 (merged)

the Eigen related files (eigenvectorbackend.hh, eigenmatrixbackend.hh and eigensolverbackend.hh) are already deprecated and have been moved to the subfolder. The eigen.hh file already exists.

mentioned in issue dune-pdelab-howto#4 (closed)

mentioned in merge request !22 (merged)

mentioned in commit acf49b8b

mentioned in commit andreas.nuessing/dune-pdelab@acf49b8b

This should be done, right?

Status changed to closed

yes, this is done.

mentioned in commit exadune/dune-pdelab@acf49b8b