Updated derivation with improved numerical stability

[seanlaw]

Pull Request Checklist

Below is a simple checklist but please do not hesitate to ask for assistance!

• Read our Contributing Guide
• Referenced a Github issue (or create one if one doesn't already exist)
• Read and reviewed all of the comments in the Github issue that you've referenced (along with cross-referenced issues/pull requests) to ensure that the issue still requires a pull request
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[gitnotebooks]

Found 1 changed notebook. Review the changes at https://app.gitnotebooks.com/stumpy-dev/stumpy/pull/1180

[seanlaw]

@NimaSarajpoor Would you mind taking a look when you have some time? Thanks in advance!

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