add runner to LinearSolver and make it accessible, allow to derive from PetscRunner for user-defined petsc solver implementation

Added init-finalize also for vectors.
Require gather/scatter operations instead of direct vector/matrix access.
Rewritten the interpolation.
Added ParallelIndexSet and DOFMapping to be used also in other backends.

Müller, Felix authored Jun 14, 2019 and Praetorius, Simon committed Jun 14, 2019

Changed ISTL solver interface to use parallel solver; Added empty communicator for sequential use of parallel solvers

In the Eigen backend, the preconditioners and direct solvers might reuse the symbolic pattern of factorization