Rename `threshold_keys`

[teubert]

Rename threshold_keys to events to align with terminology in predictor. threshold_keys will still work, but will warn. added test and updated examples

[github-actions[bot]]

Thank you for opening this PR. Each PR into dev requires a code review. For the code review, look at the following:

• [ ] Reviewer (someone other than author) should look for bugs, efficiency, readability, testing, and coverage in examples (if relevant).
• [ ] Ensure that each PR adding a new feature should include a test verifying that feature.
• [ ] All errors from static analysis must be resolved.
• [ ] Review the test coverage reports (if there is a change) - will be added as comment on PR if there is a change
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• [ ] Any added dependencies are included in requirements.txt, setup.py, and dev_guide.rst (this document)
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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.52542% with 5 lines in your changes missing coverage. Please review.

Project coverage is 83.36%. Comparing base (0980f8c) to head (1ace59a). Report is 37 commits behind head on dev.

Files	Patch %	Lines
src/progpy/prognostics_model.py	81.81%	4 Missing :warning:
src/progpy/data_models/lstm_model.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #147 +/- ## ========================================== - Coverage 83.41% 83.36% -0.06% ========================================== Files 100 100 Lines 10462 10553 +91 ========================================== + Hits 8727 8797 +70 - Misses 1735 1756 +21 ```

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