Cleanup: Matrix backends in boilerplate code
Replace deprecated ISTLMatrixBackend by BCRSMatrixBackend.
We should add to the release notes and/or to the documentation how the number of nonzeros can be cross-checked.
Replace deprecated ISTLMatrixBackend by BCRSMatrixBackend.
We should add to the release notes and/or to the documentation how the number of nonzeros can be cross-checked.