ENH: either support MultiIndex or raise an error when one is given

[u3ks]

Closes #606

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.0%. Comparing base (4037c70) to head (df2864f).
Report is 60 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #622     +/-   ##
=======================================
+ Coverage   97.4%   98.0%   +0.6%     
=======================================
  Files         26      38     +12     
  Lines       4328    6223   +1895     
=======================================
+ Hits        4214    6097   +1883     
- Misses       114     126     +12     

Files	Coverage Δ
momepy/functional/_elements.py	99.2% <100.0%> (ø)
momepy/functional/tests/test_elements.py	99.0% <100.0%> (ø)
momepy/functional/_shape.py	99.1% <80.0%> (ø)