Remove unused reverse_graph (#376)

* Deprecate reverse_graph in favor of pytools version

* remove it and clean up tests

* merge cleanup

* more comprehensive tests

* minor UsersCollector fixes, remove nonfunctional test part

UsersCollector and NUsersCollector have slightly different
behavior regarding duplicates (ignore in UsersCollector but
counted in NUsersCollector), hence the previous test failed.

* fix flake8

* Update __init__.py

* restore reverse_graph in test

* add progress indicator comment

Co-authored-by: Andreas Kloeckner <inform@tiker.net>