Add initial support for MultiVariate Variables

[aloctavodia]

This adds general support for Multivariate variables and implements Dirichlet and Multivariate Normal in particular. Both implementations omit some methods compared to univariate. This is partly because of the differences between these two types of distributions, but also because we should think about which methods of elicitation we really want to implement for Multivariate distributions and work from there. One big missing method is plot_interactive, this should be added in the following PR:

• Code style is correct (follows pylint and black guidelines)
• Includes new or updated tests to cover the new feature
• New features are properly documented (with an example if appropriate)
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)

[codecov-commenter]

Codecov Report

Merging #224 (39534e2) into main (5fe1cac) will decrease coverage by 1.42%.
The diff coverage is 68.16%.

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
- Coverage   84.41%   82.99%   -1.42%     
==========================================
  Files          26       29       +3     
  Lines        3259     3564     +305     
==========================================
+ Hits         2751     2958     +207     
- Misses        508      606      +98     

Impacted Files	Coverage Δ
preliz/internal/plot_helper_multivariate.py	48.71% <48.71%> (ø)
preliz/distributions/distributions_multivariate.py	54.32% <54.32%> (ø)
preliz/distributions/continuous_multivariate.py	97.22% <97.22%> (ø)
preliz/distributions/__init__.py	100.00% <100.00%> (ø)
preliz/tests/test_distributions.py	100.00% <100.00%> (ø)
preliz/tests/test_plots.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[aleicazatti]

Suggestions for some typos and small changes in docstrings.