Add n_points argument to x_vals.

[aloctavodia]

This also improves the docstring and set x_vals support to "restricted" to plot_pdf and plot_cdf

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.77%. Comparing base (2e7aade) to head (24b5c50).

Files	Patch %	Lines
preliz/distributions/distributions.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #357      +/-   ##
==========================================
- Coverage   82.73%   81.77%   -0.96%     
==========================================
  Files          52       52              
  Lines        5154     5157       +3     
==========================================
- Hits         4264     4217      -47     
- Misses        890      940      +50     

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