feat: added trivial hypergraph

[maximelucas]

No description provided.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (b42a04f) 90.29% compared to head (23219c1) 90.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #335      +/-   ##
==========================================
+ Coverage   90.29%   90.32%   +0.02%     
==========================================
  Files          41       41              
  Lines        2958     2966       +8     
==========================================
+ Hits         2671     2679       +8     
  Misses        287      287              

Impacted Files	Coverage Δ
xgi/generators/classic.py	96.29% <100.00%> (+1.55%)	⬆️

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[maximelucas]

More general question: our current implementation of empy_hypergraph looks more complicated than it could.
Could it not just be xgi.Hypergraph().
Any thoughts?