change Model.run so parameters always get passed

[dylanhmorris]

Previously, parameters passed would get ignored on calls to run() after the first, since run did not re-create the MCMC and NUTS objects. Since this creation operation is not expensive, I think they should just be re-created at each run() call

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.79%. Comparing base (a2bd8e4) to head (b9c0769).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #279      +/-   ##
==========================================
- Coverage   92.80%   92.79%   -0.01%     
==========================================
  Files          40       40              
  Lines         903      902       -1     
==========================================
- Hits          838      837       -1     
  Misses         65       65              

Flag	Coverage Δ
unittests	92.79% <100.00%> (-0.01%)	⬇️

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