dofOnBoundary missing for some discretization schemes

Feature request

Add dofOnBoundary function for various discretization schemes. Currently implemented for box,diamond,pq1bubble,porenetwork.

What does this feature / why does DuMux need it:

Some preconditioners require a vector of dirichlet dof's to function properly. For instance StokesSolver's preconditioner (see Test). In order to use these preconditioners a way to find dirichlet dof's needs to be introduced for various discretization schemes.

Which issue does this feature fix (if any)

Makes more preconditioners useable. StokesSolver should be usable with multidomain and staggered.