This PR implements PolarBoundaryConditions. This was originally done in #3953 with views of abstract operations (allowing reductions on windowed fields).
However, two problems arose:

• allowing views of abstract operations might be more complicated than originally thought because we need to pass the topology in the intersect index function (requires a bit of refactoring).
• the BoundaryConditions module is loaded before the Fields module, requiring quite a mess for a boundary condition which is, in the end, quite simple.

For this reason, I would like to close #3953 and just implement a simple PolarBoundaryCondition in this PR with a dedicated kernel to compute the mean of the first / last row.

I can open an issue regarding views of AbstractOperations so we can tackle it in a new fresh PR.