Add ability to generate ER hypergraphs without multiedges

[nwlandry]

This PR fixes #339.

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93.14%. Comparing base (5dc6f4f) to head (aecb125).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
xgi/generators/uniform.py	89.65%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
- Coverage   93.16%   93.14%   -0.02%     
==========================================
  Files          60       60              
  Lines        4523     4540      +17     
==========================================
+ Hits         4214     4229      +15     
- Misses        309      311       +2     

Flag	Coverage Δ
	93.14% <90.62%> (?)

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[nwlandry]

@arashbm --- does this fix the issue you raised?