Extend SDP-CBF interface to multiple visibility beams

Why now? We are getting close to implementing visibility transfer, and we should likely clarify how this works before it causes confusion

Scope: SDP, CBF Low, (CBF Mid)

Quality: Modifiability

For CBF Mid there will generally only be one visibility beam, but for CBF Low we likely want at least the option to form multiple visibility beams. This is likely just a matter of reserving a field in the SPEAD header.