Issue 29 hierarchical log pdf

[DavAug]

Partially closes #29

[codecov]

Codecov Report

Merging #66 (d4b700f) into main (45792aa) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main       #66    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           11        12     +1     
  Lines          824       926   +102     
==========================================
+ Hits           824       926   +102     

Impacted Files	Coverage Δ
erlotinib/__init__.py	100.00% <100.00%> (ø)
erlotinib/_log_pdfs.py	100.00% <100.00%> (ø)
erlotinib/_mechanistic_models.py	100.00% <100.00%> (ø)
erlotinib/_population_models.py	100.00% <100.00%> (ø)
erlotinib/_problems.py	100.00% <100.00%> (ø)

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