Store-agnostic packset algorithms.
Host merge is a named fuse then diversify then decay panel.
Default fuse is Borda (k - position). Reciprocal Rank Fusion,
CombSUM / CombMNZ, Dowdall, Kemeny-Young, Schulze, Copeland,
and Tideman are named fuse voters. Default diversify is MMR.
A Determinantal Point Process is a named diversify voter.
Default decay is off. The host reads PACKSET_FUSE,
PACKSET_DIVERSIFY, and PACKSET_DECAY. Clients do not
choose this.