Make HMMBase optional in the test suite

[gdalle]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.47%. Comparing base (9802bad) to head (b7a54ae).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/inference/baum_welch.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #111       +/-   ##
===========================================
- Coverage   90.07%   65.47%   -24.61%     
===========================================
  Files          17       17               
  Lines         504      504               
===========================================
- Hits          454      330      -124     
- Misses         50      174      +124     

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