add ~GraphToDictMapper~ UsersCollector (#189)
* add GraphToDictMapper
* remove *args, derive from CachedMapper
* rename to Userscollector, add index map functions
* flake8
* add random DAG test
* better doc
* rename in comment
* Fix docs and naming for UsersCollector, reverse_graph, tag_child_nodes
* UsersCollector: tweak constructor docs
Co-authored-by: [6~ <inform@tiker.net>