propagation, spectrum: Add hook for wraparound model for hexagonal deployments

This MR has been revised to allow the wraparound model to be defined (documented, tested, etc.) instead in the NR module, just making use of this callback for now. This is a temporary workaround for ns-3.45. See !2452 for a longer-term solution.

The original MR description:

Prevents users from border sites to have less cells in range (a.k.a. less interference), by communicating them with the opposite site. Also speeds up simulations by reducing the need for extra rings, gNBs, and UEs (which is why 3GPP usually uses it for calibration).

Still working on tests, release notes, docs, etc.