Add approximation for average clustering coefficient

This pull request follows from the discussion and code in #868 . I've changed the name of the function to match the conventions used in `cluster.py`, did cosmetic changes to the code, and added docstrings and tests. @fredzilla, could you take a look at this?

I'm still not sure if it would be better to add this function to `cluster.py` instead of adding it to the `approximation` module. Maybe it would be better to have all clustering related functions together. I have implemented approximations related to node connectivity and I'm also wondering if they should live in the approximation module or with the other connectivity stuff.