Fix issue with load_xgi_data HIF

[nwlandry]

This PR

• fixes an error where load_xgi_data can't load HIF-format files.
• Adds corresponding unit tests to make sure that this doesn't happen again.
• Renames G to H

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.43%. Comparing base (ade37fb) to head (cec3af9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #637      +/-   ##
==========================================
+ Coverage   93.36%   93.43%   +0.06%     
==========================================
  Files          64       64              
  Lines        4962     4962              
==========================================
+ Hits         4633     4636       +3     
+ Misses        329      326       -3     

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