ifft=False for whiten

[ahmadtourei]

Description

This PR makes whiten function capable of outputting in the frequency domain. This enhancement is particularly useful when whitening is performed as a preprocessing step for subsequent processes, such as computing cross-correlation.

Checklist

I have (if applicable):

• [x] referenced the GitHub issue this PR closes.
• [x] documented the new feature with docstrings or appropriate doc page.
• [x] included a test. See testing guidelines.
• [x] your name has been added to the contributors page (docs/contributors.md).
• [x] added the "ready_for_review" tag once the PR is ready to be reviewed.