[!332] Sum factorized Block Preconditioner

Merge branch 'feature/block-preconditioner' into 'master'

ref:extensions/dune-codegen Allows the symbolic extraction of the block
diagonal of a DG operator as well as the (rank 1) point diagonal of that
blockdiagonal. Sum factorized matrixfree evaluation of both of these is
implemented.

See merge request [extensions/dune-codegen!332]

  [extensions/dune-codegen!332]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/332