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