Replace PageRank dict-of-dicts iteration in graph_analytics

PageRank is currently a Python loop over a `dict` of node ranks for 16 iterations on 1,000 nodes.

Locate the graph_analytics entry point and read the current PageRank implementation, focusing on the 16-iteration loop over the dict of node ranks. Compare its behavior on the stated 1,000-node case before changing it. Done means the dict-of-dicts iteration is replaced while PageRank results remain correct and the case is faster.

Written by the indexing model from the issue text.