reorganize keth models into a module

[codecov[bot]]

Codecov Report

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

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #116 +/- ## ======================================= Coverage ? 77.34% ======================================= Files ? 38 Lines ? 4519 Branches ? 0 ======================================= Hits ? 3495 Misses ? 1024 Partials ? 0 ```

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