API: allow silencing of FutureWarnings from legacy API

[martinfleis]

This introduces an environment variable ALLOW_LEGACY_MOMEPY which, when set to True, silences all the deprecation warnings about the legacy API.

I am open to a discussion about the name of the variable but I'd say anything without a possibility of conflicts is good enough.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.0%. Comparing base (4037c70) to head (cebb597).
Report is 85 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #617     +/-   ##
=======================================
+ Coverage   97.4%   98.0%   +0.6%     
=======================================
  Files         26      38     +12     
  Lines       4328    6115   +1787     
=======================================
+ Hits        4214    5993   +1779     
- Misses       114     122      +8     

Files with missing lines	Coverage Δ
momepy/tests/test_utils.py	99.4% <100.0%> (+0.1%)	⬆️
momepy/utils.py	98.9% <100.0%> (+0.1%)	⬆️

[jGaboardi]

I think ALLOW_LEGACY_MOMEPY is good enough. Maybe to be more concise we could do USE_LEGACY_MOMEPY, but the difference is trivial.