New iterative generators

[maximelucas]

Added three iterative deterministic generators:

• pseudo-fractal simplicial complex (closes incorporate pseudofractal generative models #507)
• apollonian complex
• network geometry with flavor (NGF) (closes incorporate Network-Geometry-with-Flavor #304)

Code adapted from the "Higher-order Laplacian Renormalization" repo.

[codecov]

Codecov Report

Attention: Patch coverage is 48.75000% with 41 lines in your changes missing coverage. Please review.

Project coverage is 92.80%. Comparing base (18de14b) to head (cde2bc1).

Files with missing lines	Patch %	Lines
xgi/generators/iterative.py	48.10%	41 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #661      +/-   ##
==========================================
- Coverage   93.50%   92.80%   -0.71%     
==========================================
  Files          64       65       +1     
  Lines        5005     5085      +80     
==========================================
+ Hits         4680     4719      +39     
- Misses        325      366      +41     

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[maximelucas]

@Marconurisso @MartaMorandini Can you check again that these two functions look correct? I wrote tests only for order=2 and n_iter up to 2, can you add basic ones for another value of order if it's worth it?

[maximelucas]

I've now also added an NGF function, please check that it's correct and we should add a few simple tests.