Fix MultiLevelStaticCond with periodic BCs

This should fix issue #21 (closed), regarding the use of MultiLevelStaticCond with periodic boundary conditions in parallel. Basically, this involves forcing all periodic boundary conditions to be on the last level of the static cond. I expected doing this only for the dofs connected to other partitions would be enough, but apparently it is not since it leads to a segfault in one of the cases I tested.