Feature/networkx helpers

[PyMap]

@sablanchard, this PR includes:

1. Directed or Undirected MultiGraph parameter implementation for the ua_to_networkx method.
2. get_node_relations_by_type to get self-loop and multi-edges relations in the urbanaccess_nw object (this pretends to help users to have a better understanding before networkx object instantiation)
3. network_degree_stats to get sum and average node degree of the networkx object.
4. from_parallel_to_single_edges to go from multi to single edges.

All of them are shown in the networkx_helpers demo notebook

[PyMap]

@sablanchard I found this methods

https://github.com/gboeing/osmnx/blob/main/osmnx/utils_graph.py#L318-L418

that are used to go from parallel to single edges.

I'm proposing something similar here
https://github.com/UDST/urbanaccess/blob/feature/networkx-helpers/urbanaccess/utils.py#L657-L710

... just sharing to add some conext to the proposal.

[waddell]

Thank you for the detailed NetworkX proposal and demonstration, Federico. UDST has resumed active maintenance and reviewed this PR as part of the initial UrbanAccess triage.

The proposed helpers remain potentially useful, but this PR targets feature/networkx-converter, which was never merged into the repository's integration branch. It therefore cannot be evaluated or merged independently against the current maintenance baseline.

We will first reconcile dev, restore CI, and establish current dependency compatibility. After that, this work should be reconsidered as a focused feature proposal against the active integration branch, with tests separated from the large demonstration notebook where practical.

Tracked in #95. We are leaving the PR open for now and are not asking you to rework it until the baseline is established.