ENH: add preserve_index keyword to gdf_to_nx to allow preservation of index and order

[martinfleis]

Conversion to nx and back currently messes up the order of the rows and does not preserve index at all. This allows for a 1:1 roundtrip needed to keep the Graph objects and anything linked to index aligned.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.2%. Comparing base (4037c70) to head (e5f196a).
Report is 88 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #641     +/-   ##
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+ Coverage   97.4%   98.2%   +0.8%     
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  Files         26      38     +12     
  Lines       4328    6253   +1925     
=======================================
+ Hits        4214    6138   +1924     
- Misses       114     115      +1     

Files with missing lines	Coverage Δ
momepy/tests/test_utils.py	99.4% <100.0%> (+0.2%)	⬆️
momepy/utils.py	99.1% <100.0%> (+0.2%)	⬆️