Description

Add two helper functions to the phasor module to convert between lifetime fractional intensities and pre-exponential amplitudes. These functions will probably be moved to a lifetime module at some point.

Also rename the Cython extension module from _phasor to _phasorpy since it will contain functions not related to the phasor module.

Release note

Summarize the changes in the code block below to be included in the
release notes:

Add functions to convert between lifetime fractions and amplitudes

Checklist

• The pull request title, summary, and description are concise.
• Related issues are linked in the description.
• New dependencies are explained.
• The source code and documentation can be distributed under the MIT license.
• The source code adheres to code standards.
• New classes, functions, and features are thoroughly tested.
• New, user-facing classes, functions, and features are documented.
• New features are covered in tutorials.
• No files other than source code, documentation, and project settings are added to the repository.