Fix bayes prior bug

[vschutze-alt]

This PR fixes a bug in the uniform prior, which if given max and min for all parameters as floats would generate the wrong size samples from the sample method. It needs to be an array of values for the distribution to know the correct parameter size.

It also adds the n_parameters attribute to the pdf model base class, which is currently used by blackjax and is in general useful to have.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (47ea395) to head (ce6ab9a).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   94.96%   95.03%   +0.06%     
==========================================
  Files          31       31              
  Lines        1649     1652       +3     
==========================================
+ Hits         1566     1570       +4     
+ Misses         83       82       -1     

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