Add faster Exponential

[rohanbabbar04]

Description

• Add a class Exponential with all methods.
• Update test in test_scipy.py.

Checklist

• 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)

[aloctavodia]

Thanks!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.63636% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 82.85%. Comparing base (99e7413) to head (8795c96).
Report is 2 commits behind head on main.

Files	Patch %	Lines
preliz/distributions/exponential.py	89.28%	9 Missing ⚠️
preliz/internal/special.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #355      +/-   ##
==========================================
+ Coverage   82.76%   82.85%   +0.09%     
==========================================
  Files          50       51       +1     
  Lines        5048     5099      +51     
==========================================
+ Hits         4178     4225      +47     
- Misses        870      874       +4     

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