Functional API for applying transition matrix

[gdalle]

• Implement a function predict_previous_state! mirroring predict_next_state!. Users can theoretically override both instead of transition_matrix and have a functional forward/backward (Viterbi is still unsolved).
• Comment out Enzyme test in docs due to latest release

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.27%. Comparing base (a370596) to head (e1a35af).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   95.22%   95.27%   +0.04%     
==========================================
  Files          18       18              
  Lines         503      508       +5     
==========================================
+ Hits          479      484       +5     
  Misses         24       24              

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