Update `AdaptationOutput` to make `X` into an attribute

[kachayev]

• cleanup code for adaptation output processing by moving X to an attribute instead of keys
• fix documentation for reweight adapters ('returns' section of docstrings)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.58%. Comparing base (ad285c8) to head (2e8a1e2).

Files	Patch %	Lines
skada/tests/test_subspace.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #106 +/- ## ========================================== - Coverage 89.61% 89.58% -0.03% ========================================== Files 41 41 Lines 2850 2852 +2 ========================================== + Hits 2554 2555 +1 - Misses 296 297 +1 ```

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