Boundary covering-cube transports

Compatibility import for the six containment-based transports. Their implementations live in BoundaryPoincareWindow, BoundaryTransports, and BoundaryGradH, where the geometry-based interfaces derive containment and apply the same results. The original public declaration names and both public imports are preserved.

Ported from scottnarmstrong/Superdiffusion at 7f20476385d4af418f110303085ad0740c9eba46.