domain decomposition methods as preconditioners

As far as I can tell, preconditioning using domain decomposition methods is not something that seems to be supported. Is this something that is within the scope of `Ferrite.jl`? Of course there are C implementations that are available through PETSc, but I think a pure Julia implementation would be beneficial. As an example, perhaps a (restricted) additive Schwarz method would be worth contributing? See [PETSc: PCASM](https://petsc.org/main/manualpages/PC/PCASM/) for details.