Warn for Null values in series when calling event detection

[jarq6c]

Warn the user if event detection is called on a series with null values. Closes #51

Additions

• UserWarning when calling detrend_series that contains null values

Removals

• None

Changes

• None

Testing

1. Warning is tested.

Notes

• Docs/README still recommends to fill null values.

Todos

• None

Checklist

• PR has an informative and human-readable title
• PR is well outlined and documented. See #12 for an example
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (see CONTRIBUTING.md)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output) using numpy docstring formatting
• Placeholder code is flagged / future todos are captured in comments
• Reviewers requested with the Reviewers tool ➡️

[jarq6c]

Passed all tests.