Implement that neither Dirichlet boundary values nor fixed pressure values (cf.
setFixedCellin dumux/freeflow/navierstokes/staggered/localresidual.hh) are degrees of freedom. Make sure that whenever those non-dof Dirichlet values are part of the stencil they are considered in a source-term manner.
Give linear operators to SchurComplementPreconditioner, SchurApproximate and SchurComplement instead of matrices.
Find a better name for InnerSolver