Add a RightFolded topology

[simone-silvestri]

this topology, for the moment, is particular to the tripolar grid. It works in the same exact way as a RightConnected topology except for two differences:

• the topology-aware operators (used for the split explicit free surface) are different because they encode the fold in the operator
• the right boundary conditions associated with a RightFolded are ZipperBoundaryConditions

This PR just introduces the topology and the associated topology-aware operators to have the TripolarGrid working. The implementation of correct default boundary conditions for the RigthFolded topology is left for a successive PR