CFSv2: deprecate --IFO (only used for v1 SFTs)

As discussed in !178 (merged) we should deprecate the --IFO option which says "(useful for single-IFO v1-SFTs only!)", is confusing to see side-by-side with the new --IFOs introduced in that MR, and is described by @reinhard-prix as "by now misleading at best".