return frequency in signal.signal_filter._signal_filter_sanitize as scalar

[MarcBalle]

Description

In order to avoid the DeprecationWarning thrown by NumPy (DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)) , the freq variable should be returned as a scalar and not as a list

Proposed Changes

Change the freqs variable from list to a scalar when filter is either lowpass or highpass

Checklist

Here are some things to check before creating the PR. If you encounter any issues, do let us know :)

• I have read the CONTRIBUTING file.
• My PR is targeted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.
• I have added the newly added features to News.rst (if applicable)

[welcome]

Thanks for opening this pull request! We'll make sure it's perfect before merging 🤗
Make sure to read the contributing guide. Also, if you think that your contribution is worthy of it, you can consider adding yourself to the Contributors list (feel free to ask us if you have any doubts).

[welcome]

Congrats on merging your first pull request! 🎉🍾 We're looking forward to your next one!