Reuse conditional average outcome estimates for X-Learner pseudo outcome

[kklein]

This refactoring does not resolve https://github.com/Quantco/metalearners/issues/80 but it might make resolving it easier.

Related to https://github.com/Quantco/metalearners/issues/82

Checklist

• [ ] Added a CHANGELOG.rst entry

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.42%. Comparing base (d863df1) to head (7941aef). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #83 +/- ## ======================================= Coverage 94.42% 94.42% ======================================= Files 15 15 Lines 1793 1793 ======================================= Hits 1693 1693 Misses 100 100 ```

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[kklein]

FYI @MatthiasLoefflerQC