Estimating the Average Clustering Coefficient of a Graph

This algorithm computes the average clustering coefficient for a graph, using an established algorithm from network analysis.

Here, I've added only one file, networkx/algorithms/approximation/clustering_coefficient.py. In it, I have defined the function "approx_clustering_coeff", which takes two arguments:
  G: The graph whose average clustering coefficient we want estimated.
  numTrials: The number of times to test to find triangles.

Please let me know if my code is "up to snuff", or if there is anything I can do to make it better!