Split fit in fit_all_nuisance and fit_all_treatment

[FrancescMartiEscofetQC]

This PR splits the fit method of each MetaLearner into fit_all_nuisance and fit_all_treatment. The goal is to make it easier for the user to "intervene" while fitting between the two steps.

Checklist

• Added a CHANGELOG.rst entry

[codecov]

Codecov Report

Attention: Patch coverage is 87.80488% with 5 lines in your changes missing coverage. Please review.

Project coverage is 94.85%. Comparing base (348460d) to head (e7bfa77).
Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
metalearners/metalearner.py	75.00%	2 Missing ⚠️
metalearners/xlearner.py	81.81%	2 Missing ⚠️
metalearners/drlearner.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   95.01%   94.85%   -0.17%     
==========================================
  Files          15       15              
  Lines        1504     1534      +30     
==========================================
+ Hits         1429     1455      +26     
- Misses         75       79       +4     

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[kklein]

LGTM :)