[backport] Merge branch 'feature/fix-istl-blockdiagonal' into 'master'
[Bugfix][istl] Fix compilation error in ISTL block diagonal code
As pointed out by Nagaiah Chamakuri, we missed a BlockVector
to Dune::BlockVector
conversion in an untested template in
the ISTL code, causing compilation failure when using the block
diagonal helper code.
See merge request !157 (merged)