Periodic Immersed Boundaries

[danieljvickers]

There is currently no concept of periodicity for immersed boundary generation when using periodic boundary conditions. This means that moving immersed boundaries that cross the boundary will have errors, as only cells on the side of the grid with the centroid will be included in the calculations of the boundary markers.

A PR for this needs to resolve this issue. Ideally a test case of a moving immersed boundary crossing the period boundary diagonally (as that will test the periodicity wrapping around two dimensions at once) should be added.