Periodic boundary conditions

[einola]

We need periodic boundary conditions for

• Idealised setups
• For regional setups with a zonal periodicity (e.g. the southern ocean)
• For global "tri-polar" setup with a fold at the north pole

The main tasks are to

• [ ] Implement periodicity in the dynamics and advection code
• [ ] Make sure information about which borders are periodic flows between the restart file and the dynamics

[einola]

We'll do this as part of the MPI communication.