[boxAssembly] Setting Dirichlet BCs fails for explicit box assembly

For box, Dirichlet BCs are applied in BoxLocalAssemblerBase

            res[scvI.dofIndex()][eqIdx] = this->curElemVolVars()[scvI].priVars()[pvIdx] - dirichletValues[pvIdx];
            for (const auto& scvJ : scvs(this->fvGeometry()))
            {
                jac[scvI.dofIndex()][scvJ.dofIndex()][eqIdx] = 0.0;
                if (scvI.localDofIndex() == scvJ.localDofIndex())
                    jac[scvI.dofIndex()][scvI.dofIndex()][eqIdx][pvIdx] = 1.0;
            }

The offending line is jac[scvI.dofIndex()][scvJ.dofIndex()][eqIdx] = 0.0; since in the explicit case, there are only diagonal entries in Jacobian. A dune-istl error will be raised.