split pymc_experiments into smaller files

[jpreszler]

No functional code changes, just splitting into smaller files.

This relates to discussion #253 .

[codecov]

Codecov Report

Attention: Patch coverage is 70.57471% with 128 lines in your changes missing coverage. Please review.

Project coverage is 74.97%. Comparing base (133b987) to head (20fabbc).
Report is 452 commits behind head on main.

Files	Patch %	Lines
...alpy/pymc_experiments/difference_in_differences.py	61.00%	39 Missing ⚠️
causalpy/pymc_experiments/pre_post_fit.py	59.77%	35 Missing ⚠️
causalpy/pymc_experiments/pre_post_negd.py	67.14%	23 Missing ⚠️
...salpy/pymc_experiments/regression_discontinuity.py	72.83%	22 Missing ⚠️
causalpy/pymc_experiments/experimental_design.py	57.14%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #254      +/-   ##
==========================================
+ Coverage   73.89%   74.97%   +1.08%     
==========================================
  Files          19       25       +6     
  Lines        1149     1199      +50     
==========================================
+ Hits          849      899      +50     
  Misses        300      300              

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[drbenvincent]

So this was a good idea, but has been superseded by #381