Move computation of DOCA{,Chi2} related quantities from 'combinations' to functors which use 'combinations'

- implement a generic interface which combinations must support to allow this:
  - get<N>()   - to get the N-th candidate
  - transform_reduce( transformer, reducer ) - calls transformer on
                                               each element, and uses
                                               reducer to aggregate the
                                               result
  - pairwise_transform_reduce  - as above, but loops over all pairs, so
                                transformer must be a binary operator
  - pairwise_none_of - return whether all pairs pass the provided predicate