[WIP] Classes for discrete PMF vectors

[dylanhmorris]

Closes #490

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.59%. Comparing base (b7a493e) to head (6506c24).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pyrenew/randomvariable/pmf.py	58.82%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #696      +/-   ##
==========================================
- Coverage   97.43%   96.59%   -0.85%     
==========================================
  Files          51       52       +1     
  Lines        1523     1558      +35     
==========================================
+ Hits         1484     1505      +21     
- Misses         39       53      +14     

Flag	Coverage Δ
unittests	96.59% <60.00%> (-0.85%)	⬇️

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[github-actions]

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