Make default STEP_METHODS a list that can be modified

[ricardoV94]

Description

Having as a tuple is a bit cumbersome and makes it seem like it should not be modified. However, until we have a more proper system (disptach based probably), this is the API for other packages (including BART, pymc-experimental) to define default samplers.

This PR makes it a list (and over-engineers a fixture for the test :D, happy to remove if you think it's silly)

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.29%. Comparing base (22e8f0b) to head (68457f2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7231   +/-   ##
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  Coverage   92.29%   92.29%           
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  Files         101      101           
  Lines       16892    16892           
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  Hits        15590    15590           
  Misses       1302     1302           

Files	Coverage Δ
pymc/step_methods/__init__.py	100.00% <100.00%> (ø)

[jessegrabowski]

Looks nice and simple. Trying to think about corner cases where a simple list wouldn't be good enough for holding the step methods. Does it matter if a user appends the same sampler to the list multiple times? Or is the ordering of the list important anywhere downstream?

[ricardoV94]

The order should not be important, except that for RNG reproducibility it always ends up being important. Repeated entries shouldn't be a problem