Fix #449: rewrite causal-flow algorithm iteratively.

[matulni]

This commit replaces the recursion call in the causal flow algorithm by a while loop. This rewriting prevents the RecursionError reported in #449 for linear graphs with more than 1000 nodes.

Example

import timeit

import networkx as nx
from graphix.fundamentals import Plane
from graphix.opengraph import OpenGraph


def create_line_og(n_nodes: int) -> OpenGraph[Plane]:
    if n_nodes < 2:
        raise ValueError("n_nodes must be larger than 1.")

    graph: nx.Graph[int] = nx.Graph([(i, i + 1) for i in range(n_nodes - 1)])
    input_nodes = [0]
    output_nodes = [n_nodes - 1]
    measurements = dict.fromkeys(range(n_nodes - 1), Plane.XY)
    return OpenGraph(graph, input_nodes, output_nodes, measurements)

def compute_flow(n_nodes: int):
    og = create_line_og(n_nodes)
    def _compute_flow():
        og.extract_causal_flow()
    return _compute_flow

n_nodes = 5000
t = timeit.Timer(compute_flow(n_nodes))
print(t.timeit(5))  # 6.928544000002148

TODO

Update CHANGELOG

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.29%. Comparing base (daf62cb) to head (6810248).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
- Coverage   88.29%   88.29%   -0.01%     
==========================================
  Files          44       44              
  Lines        6535     6533       -2     
==========================================
- Hits         5770     5768       -2     
  Misses        765      765              

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[thierry-martinez]

Thanks for this nice change! I just have a few minor comments.

[emlynsg]

Agree with Thierry's comments. Nice fix and clean tests!

[thierry-martinez]

This comment is no longer relevant. The whole expression can now fit on the same line.

[matulni]

Ahh sorry, I misunderstood which comment you referred to. Done in 115d800