Add N(>2)-particle combinatorics, and an example implementing the ADL benchmarks

I wanted to submit a bamboo implementation to the ADL benchmarks, but noticed that combinatorics with N>2 particles was not implemented, so added that (also a nice excuse to read up on parameter packs ;-) ).

Note: there is one API change, in op.combine the sameIdxPred keyword argument is replaced by samePred (since !70 (merged) getting the index is trivial); I don't think it was much used yet, and this allows to also make e.g. sorted combinations (I also made it more explicit in the documentation how this works).