Add pick command for reduce

Copied from #267:

scikit-hep/boost-histogram#296
suggests that one should be able to pick individual bin indices from a category axis. The suggested command is `pick(index1, index2, ...)`, as suggested by @henryiii

The idea is good, but the implementation cost is not negligible. `pick` would only work for unordered axes like category. A new kind of reduce constructor must be defined and a corresponding reduce implementation must be written.