Modularize model in preparation for fitting real NHSN data

[dylanhmorris]

Major change

• Create helper RVs for the latent infection process and the three observation processes, and produce a PyrenewHEWModel

Minor changes

• Tweak some variable names for clarity, rename PyrenewHEWModel class to title case to accord with general Python class naming conventions.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 94 lines in your changes missing coverage. Please review.

Project coverage is 15.87%. Comparing base (1fb4c2e) to head (9bdf638).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pyrenew_hew/pyrenew_hew_model.py	0.00%	85 Missing ⚠️
pipelines/build_pyrenew_model.py	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
- Coverage   16.48%   15.87%   -0.62%     
==========================================
  Files          21       21              
  Lines        1480     1537      +57     
==========================================
  Hits          244      244              
- Misses       1236     1293      +57     

Flag	Coverage Δ
hewr	47.68% <ø> (ø)
pipelines	0.00% <0.00%> (ø)
pyrenew_hew	1.65% <0.00%> (-0.24%)	⬇️

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[sbidari]

Looks good to me!