split pymc_experiments into smaller files

[jpreszler]

No functional code changes, just splitting into smaller files.

This relates to discussion #253 .

[codecov]

Codecov Report

Merging #254 (20fabbc) into main (133b987) will increase coverage by 1.08%.
The diff coverage is 70.57%.

@@            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              

Files	Coverage Δ
causalpy/pymc_experiments/__init__.py	100.00% <100.00%> (ø)
causalpy/pymc_experiments/instrumental_variable.py	100.00% <100.00%> (ø)
causalpy/pymc_experiments/experimental_design.py	57.14% <57.14%> (ø)
...salpy/pymc_experiments/regression_discontinuity.py	72.83% <72.83%> (ø)
causalpy/pymc_experiments/pre_post_negd.py	67.14% <67.14%> (ø)
causalpy/pymc_experiments/pre_post_fit.py	59.77% <59.77%> (ø)
...alpy/pymc_experiments/difference_in_differences.py	61.00% <61.00%> (ø)