Draft: Implement a seven-body combiner

Implement a seven-body combiner, primarily used in the search for very rare decays like tau leptons decaying to seven muons. This solves the problem of having multiple candidates in the lines targeting these decays due to the combination of intermediate (auxiliar) particles in such a way that the muons could be swapped among them in such a way that the candidates are considered "different" but they actually refer to the same physical candidate.

To be merged with Moore!3394 and lhcb-datapkg/ParamFiles!87.