warn if loading an xml file generated by OSMnx itself

[gboeing]

Users sometimes attempt to re-load XML files from save_graph_xml back into OSMnx for later work using graph_from_xml (e.g. #1023), though the documentation warns them not to. Due to format limitations in save_graph_xml, this use case is not supported. This PR makes this more explicit by a user warning if an XML file loaded by graph_from_xml as generated by OSMnx.

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.99%. Comparing base (6130d55) to head (40f6498).
Report is 917 commits behind head on main.

Files with missing lines	Patch %	Lines
osmnx/osm_xml.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1031      +/-   ##
==========================================
- Coverage   98.03%   97.99%   -0.04%     
==========================================
  Files          24       24              
  Lines        2438     2443       +5     
==========================================
+ Hits         2390     2394       +4     
- Misses         48       49       +1     

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