Constraint from constraint matrix

It would be nice to simplify https://github.com/Ferrite-FEM/Ferrite.jl/blob/084a7c9d967047efa986dcb3ac0668033e023084/docs/src/literate/stokes-flow.jl#L316-L324 by either implementing a new constraint type `MatrixConstraint(C, f)` (or some other name) which you can add directly to the constraint handler, or at least have a helper function that computes the affine constraints. In particular this would be useful if `C` has multiple rows, since then different dofs should be selected as the constrained one.