add tri-polar periodic boundary condition

[TomMelt]

relates to #43

If this issue is fixed, it will also fix #43.

This is part of a larger issue to implement tri-polar grids into nextsim: nextsimhub/nextsimdg#1081

The following parts need to be implemented:

1. First, we need to manually decompose the domain into 2 equally-size domains down the north-south line
2. If along the top or bottom edge, we need to calculate the new periodic neighbor based on the opposite side (see diagram in tri-polar support nextsimdg#1081)