Additional Dirichlet constraints

I would be nice to be able to set Dirichlet constraints for any dof in the problem. The assembler could ask the problem if it defines additional constraints (constexpr bool func or by member function inspection (probably too magical)) and if yes implement these constraints after the assembly.

This feature could fix e.g. #704 (closed). However I'm having problems to come up with other used cases, maybe some overlapping domain decomposition stuff.