Access posterior data by key

[michaelosthege]

• Access posterior group by key (better typing)
• Test pipeline passed with PyMC v6
• Avoiding InferenceData import

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.58%. Comparing base (cc054c0) to head (8b353f5).

Files with missing lines	Patch %	Lines
bletl/growth.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #92   +/-   ##
=======================================
  Coverage   93.58%   93.58%           
=======================================
  Files          11       11           
  Lines        1248     1248           
=======================================
  Hits         1168     1168           
  Misses         80       80           

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