Periodic boundary conditions for the southern ocean

[einola]

We need to have periodic boundary conditions active in the full model to be able to run the southern ocean part of the global domain. For this, we need the following:

• Periodic boundary conditions implemented in the dynamics/DG code
• The ability to use the ORCA/NEMO southern hemisphere grid
• Make sure there's nothing in the way of using PBC in the full model

Much of this is done, but I'm unsure how mature and tested everything is.

This should link to a general issue of using ORCA/NEMO grids in neXtSIM, but I can't find one.