Add is_trained accessor & conditional retraining to the pipeline

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.00%. Comparing base (81bccfa) to head (8f2134f). Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/pipeline/__init__.py	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #524 +/- ## ======================================= Coverage 92.99% 93.00% ======================================= Files 101 101 Lines 7008 7044 +36 ======================================= + Hits 6517 6551 +34 - Misses 491 493 +2 ```

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