Add generic observation processes which combine the convolution with the noise model.

[cdc-mitzimorris]

This PR adds work that was done in https://github.com/cdcent/cfa-pyrenew-hierarchical/pull/4 to PyRenew.

It adds the base observation process class, concrete implementations for Count processes and the abstract base class for Measurement processes, together with unit tests and two new tutorials for count and measurement observation processes respectively.

Once this PR and the work done in https://github.com/cdcent/cfa-pyrenew-hierarchical/pull/5 have been added to PyRenew, subsequent PRs will deprecate unused features and harmonize the documentation and tutorials.

[codecov]

Codecov Report

❌ Patch coverage is 97.81421% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.18%. Comparing base (02446c5) to head (df9f76e).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pyrenew/observation/count_observations.py	94.91%	3 Missing ⚠️
pyrenew/observation/noise.py	98.38%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   96.98%   97.18%   +0.19%     
==========================================
  Files          42       47       +5     
  Lines        1094     1277     +183     
==========================================
+ Hits         1061     1241     +180     
- Misses         33       36       +3     

Flag	Coverage Δ
unittests	97.18% <97.81%> (+0.19%)	⬆️

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[github-actions]

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[dylanhmorris]

Thanks @cdc-mitzimorris. Final fix needed for NaN-padding question, all other changes can be deferred to future PRs.

[cdc-mitzimorris]

@dylanhmorris - all requested changes made.

[dylanhmorris]

Thanks @cdc-mitzimorris! Left suggestions that should fix the tutorial rendering. Apologies for failing to make those along with the test changes in my last review. Please merge when ready!