Allow for periodicity in domains which are not simply connected

[p-costa]

Right now periodicity won't be detected for certain domains where it may be prescribed, such as the flow around a periodic array of obstacles. Periodicity will not be detected due to a lack of connectivities of inner boundaries.

This can be relaxed so that periodicity is detected not only for a cyclic succession of connectivity BCs, but also for cyclic BCs which may be interrupted by a 'hole'.